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Gamba grass (Andropogon gayanus Kunth.) is a highly invasive, naturalised Weed of National Significance in Australia due to its economic, environmental and social impacts. It outcompetes native pastures and fuels intense fires in northern Australian rangelands. To aid management of current infestations and to better understand its potential distribution, this study determined the germination response of gamba grass under a range of constant (13°C−48°C) and alternating (11/7°C–52/42°C) temperature regimes and quantified the potential longevity of soil seed banks. The effect of different soil types, levels of pasture cover and burial depths on seed longevity was investigated in the Dry Tropics of northern Queensland. Germination of gamba grass occurred under a wide range of both constant (17°C−39°C) and alternating day/night temperatures (16/12°C–47/39°C), although the level of germination declined at the lower and higher temperature ranges. At the cooler temperatures, seed viability was not affected, but seeds went into a state of dormancy. The highest level of seed viability was recorded at the lowest constant temperature regime (13°C) and at the two lowest alternating temperatures (11/7°C and 16/12°C). A gradual but variable decline in viability occurred thereafter with increasing temperatures. At the higher temperature range (e.g. constant temperatures of 39°C−43°C and alternating temperatures of 47/39°C) both dormancy and loss of seed viability were occurring, but once alternating and constant temperatures reached above 47/39°C and 43°C all seeds were rendered unviable after 9 and 6 weeks respectively. In the Dry Tropics of northern Queensland, viability of seeds was

Stevia ovata Willd. is an invasive weed that has become naturalised in northern Queensland, Australia. To aid management of current infestations, this study evaluated seed germination under a range of constant (13−48°C) and alternating (11/7 to 52/42°C) temperature regimes and quantified the potential longevity of soil seed banks. The effect of different soil types, levels of pasture cover and burial depths on seed longevity was investigated in both the dry- and wet-tropics of North Queensland. Germination of S. ovata occurred under a wide range of both constant (13−39°C) and alternating day/night temperatures (16/12 to 52/42°C), but optimum conditions ranged between 24 and 27°C and 24/20 and 37/31°C respectively. As temperatures declined below the optimum, an increasing proportion of seeds went into a state of enforced dormancy. In contrast, higher than optimum temperatures caused a proportion of seeds to lose viability. Differential responses in seed longevity of S. ovata occurred between the two experimental sites. In the wet-tropics, seed viability was

Cascabela thevetia (L.) Lippold (Apocynaceae), commonly known as Captain Cook tree or yellow oleander, has established large infestations in riparian areas along several creeks and rivers in northern Queensland. To better understand the ecology of C. thevetia and the implications for its spread and management, this study reports seven experiments related to the seed ecology of its yellow and peach biotypes. We quantified its germination response to ambient (Experiment 1a and 1b), alternating and constant temperature (Experiment 2a and 2b) regimes and exposure to different light conditions (Experiment 3). Seed longevity under two soil types, two levels of pasture cover and three burial depths was also determined (Experiment 4a and 4b). Both loose seeds and seeds still within pods (kernels) of the two biotypes of C. thevetia were able to germinate in all months of the year in northern Queensland, irrespective of the large differences in monthly ambient temperatures experienced at the Charters Towers study site. Both biotypes also germinated across a wide range of alternating day/night temperatures from 16/12°C to 47/37°C and constant temperatures from 17°C to 44.0°C. Germination of the two biotypes was significantly greater (4-fold) and faster (7 days earlier) under shade than under natural light conditions. Over all biotypes, soil types, levels of pasture cover and burial depths, no seeds of C. thevetia remained viable after 2 years: longevity was much less in many circumstances. The results demonstrate that C. thevetia seeds can germinate over a wide temperature range, whereas the ability of seed to remain viable at low temperatures highlights the potential for expansion of its current potential distribution towards southern latitudes of the Australian continent. Across all experimental conditions, the yellow biotype displayed superior seed germination and viability traits compared with the peach biotype. Seed banks of the peach and yellow biotypes of C. thevetia are short-lived (2 years), which may be exploited when developing management strategies to reduce its impacts.

Cascabela thevetia (L.) Lippold is a garden ornamental in northern Australia and two biotypes, the peach and the yellow, are recognised. In some areas it has naturalised and now has environmental and economic effects. As part of a broader research program into the ecology of C. thevetia, a field study was undertaken in northern Queensland to quantify seed fall and seed predation (by avian wildlife) of the peach biotype. The amount of twigging caused by birds while they fed on the seeds and the level of litter production were also recorded. Seed fall, seed predation, twigging and litter production occurred in all months of the year. Seed fall increased slowly over late spring and summer before peaking in mid- to late autumn (April–May) and then declining until October. Mean (± s.e.m.) estimated total annual seed fall was 19140 ± 2880 and 17030 ± 2930 seeds ha–1 in the first and second years respectively. Seed predation by birds was substantial, with 57% of all seeds predated. Birds also chewed an average of 600 twigs ha–1 year–1. Litter production varied from 430 to 950 kg dry weight (DW) ha–1 month–1. In total, 7900 ± 640 and 7390 ± 1420 kg (DW) litter was produced during the first and second years respectively. Although seed production of C. thevetia is less than a lot of other rangeland weeds, seed predation by birds further reduces the number of seeds entering the soil seed bank. The stem damage that occurred in conjunction with seed predation contributed to overall litter production and warrants further investigation in terms of its effect on plant growth.

Chinee apple (Ziziphus mauritiana Lam.) is a thorny tree that is invading tropical woodlands of northern Australia. The present study reports three experiments related to the seed dynamics of chinee apple. Experiment 1 and 2 investigated persistence of seed lots under different soil types (clay and river loam), levels of pasture cover (present or absent) and burial depths (0, 2.5, 10 and 20 cm). Experiment 3 determined the germination response of chinee apple seeds to a range of alternating day/night temperatures (11/6°C up to 52/40°C). In the longevity experiments (Expts 1 and 2), burial depth, soil type and burial duration significantly affected viability. Burial depth had the greatest influence, with surface located seeds generally persisting for longer than those buried below ground. Even so, no viable seeds remained after 18 and 24 months in the first and second experiment, respectively. In Expt 3 seeds of chinee apple germinated under a wide range of alternating day/night temperatures ranging from 16/12°C to 47 /36°C. Optimal germination (77%) occurred at 33/27°C and no seeds germinated at either of the lowest (11/6°C) or highest (52/40°C) temperature regimes tested. These findings indicated that chinee apple has the potential to expand its current distribution to cooler areas of Australia. Control practices need to be undertaken for at least two years to exhaust the seed bank.

The development of irrigated pastures in the Sudan is still at its infancy. The present study determined the influence of 2 cutting heights (2.5 and 5.0 cm) and 4 cutting frequencies (4-, 6-, 8-, and 10-week) on forage yield as dry matter and forage sucrose of Rhodes grass (Chloris gayana Kunth. cv. Massaba) and Ruzi grass (Brachiaria ruziziensis Germain et Everard cv. Congo Signal) under irrigated conditions. Forage yield interactions involving species X cutting height X cutting frequency were significant. These yield interactions were attributed to differences in growth period and growth rhythm of the two species. On average, Rhodes exhibited a short growth period (13 months) compared to Ruzi grass (20 months). Forage yield and forage sucrose were, on average, superior in Ruzi compared to Rhodes grass. Forage yield and forage sucrose averaged 5.2 t ha−1 and 7.2 % fw sucrose in Ruzi compared to 2.8 t ha−1 and 6.9 % fw sucrose in Rhodes grass. Forage yield and forage sucrose of both species were negatively correlated with cutting frequency. The highest forage yield may be obtained at an 8-week cutting frequency in Rhodes (45.6 t ha−1 yr−1) compared to a 10-week cutting frequency in Ruzi grass (88 t ha−1 yr−1). Results suggest that very high forage yields as dry matter and forage sucrose of Rhodes and Ruzi grass may be possible under irrigated conditions in the Sudan at a cutting frequency of between 8 to 10 weeks.

Careful cultural management of field crops has become imperative in countries where chemical measures are not economically feasible. The occurrence of the endemic parasitic witchweed [Striga hermonthica (Del.) Benth] in production fields of sorghum-sundangrass hybrid [Sorghum bicolor (L.) Moench×S. sudanense Stapf. cv. Piper], Pioneer brand hybrid 988 (coded P988), is one example that requires such procedures. Study objectives were to relate : (i) P988 seed size (large and small) to witchweed germination, P988 root and shoot length, and weight and (ii) P988 seed size and harvest growth stage to P988 greenchop yield and sucrose content at three initial and subsequent P988 harvests at boot stage, late bloom stage, and maturity in witchweed-infested fields (...)

Delaying harvest of P988 from the boot stage to the maturity stage significantly reduced 1) forage greenchop yield of P988 grown on Striga- infested soil at 1st and 3rd harvest of July sowing and at 3rd harvest of October sowing, 2) root dry weight of P988 grown on Striga-infested soil at 3rd harvest of July and October sowing, and 3) forage sucrose of P988 grown on Striga-infested soil at 3rd harvest of July sowing and at 1st and 3rd harvest of October sowing. Reductions in forage greenchop yield of P988 with successive harvests were accelerated by Striga infestations. The root dry weight was significantly greater from P988 plants grown from small compared to large seed. Delaying harvest of P988 to the maturity stage was associated with significant increases in number of open Striga flowers at 1st and 2nd harvest from October planting and in Striga shoot dry weight at 1st harvest of July and 1st and 2nd harvest of October planting. These increases were positively correlated with forage sucrose of P988. Results suggest that optimal forage greenchop yield on Striga-infested soil may be obtained from P988 plants grown from small seed and harvested at the boot stage.

Understanding the reproductive biology of Calotropis procera (Aiton) W.T. Aiton, an invasive weed of northern Australia, is critical for development of effective management strategies. Two experiments are reported on. In Experiment 1 seed longevity of C. procera seeds, exposed to different soil type (clay and river loam), pasture cover (present and absent) and burial depth (0, 2.5, 10 and 20 cm) treatments were examined. In Experiment 2 time to reach reproductive maturity was studied. The latter experiment included its sister species, C. gigantea (L.) W.T. Aiton, for comparison and two separate seed lots were tested in 2009 and 2012 to determine if exposure to different environmental conditions would influence persistence. Both seed lots demonstrated a rapid decline in viability over the first 3 months and declined to zero between 15 and 24 months after burial. In Experiment 1, longevity appeared to be most influenced by rainfall patterns and associated soil moisture, burial depth and soil type, but not the level of pasture cover. Experiment 2 showed that both C. procera and C. gigantea plants could flower once they had reached an average height of 85 cm. However, they differed significantly in terms of basal diameter at first flowering with C. gigantea significantly smaller (31 mm) than C. procera (45 mm). On average, C. gigantea flowered earlier (125 days vs 190 days) and set seed earlier (359 days vs 412 days) than C. procera. These results suggest that, under similar conditions to those that prevailed in the present studies, land managers could potentially achieve effective control of patches of C. procera in 2 years if they are able to kill all original plants and treat seedling regrowth frequently enough to prevent it reaching reproductive maturity. This suggested control strategy is based on the proviso that replenishment of the seed bank is not occurring from external sources (e.g. wind and water dispersal).

Cascabela thevetia (L.) Lippold (Apocynaceae) is an invasive woody weed that has formed large infestations at several locations in northern Australia. Understanding the reproductive biology of C. thevetia is vital to its management. This paper reports results of a shade house experiment that determined the effects of light conditions (100% or 30% of natural light) and plant densities (one, two, four or eight plants per plot) on the growth, time to flowering and seed formation, and monthly pod production of two C. thevetia biotypes (peach and yellow). Shaded plants were significantly larger when they reached reproductive maturity than plants grown under natural light. However, plants grown under natural light flowered earlier (268 days compared with 369 days) and produced 488 more pods per pot (a 5-fold increase) over 3 years. The yellow biotype was slightly taller at reproductive maturity but significantly taller and with significantly greater aboveground biomass at the end of the study. Both biotypes flowered at a similar time under natural light and low plant densities but the yellow biotype was quicker to seed (478 versus 498 days), produced significantly more pods (364 versus 203 pods) and more shoot growth (577 g versus 550 g) than the peach biotype over 3 years. Higher densities of C. thevetia tended to significantly reduce the shoot and root growth by 981 g and 714 g per plant across all light conditions and biotypes over 3 years and increase the time taken to flower by 140 days and produce seeds by 184 days. For land managers trying to prevent establishment of C. thevetia or to control seedling regrowth once initial infestations have been treated, this study indicates that young plants have the potential to flower and produce seeds within 268 and 353 days, respectively. However, with plant growth and reproduction most likely to be slower under field conditions, annual surveillance and control activities should be sufficient to find and treat plants before they produce seeds and replenish soil seed banks. The most at-risk part of the landscape may be open areas that receive maximum sunlight, particularly within riparian habitats where plants would consistently have more favourable soil moisture conditions.

Bellyache bush (Jatropha gossypiifolia L.) is an invasive weed that poses economic and environmental problems in northern Australia. Competition between pasture and bellyache bush was examined in North Queensland using combinations of five pasture treatments (uncut (control); cut as low, medium, and high pasture; and no pasture) and four bellyache bush densities (0, 2, 6 and 12 plants m−2) in a buffel grass (Cenchrus ciliaris L.) dominated pasture. The pasture treatments were applied approximately once per year but no treatments were applied directly to the bellyache bush plants. Measurements of bellyache bush flowering, seed formation, and mortality were undertaken over a 9-year period, along with monitoring the pasture basal cover and plant species diversity. Maximum flowering rates of bellyache bush occurred after 9 years (97%) in plots containing no pasture, with the lowest rates of 9% in uncut control plots. Earliest flowering (322 days after planting) and seed formation (411 days) also occurred in plots with no pasture compared with all other pasture treatments (range 1314–1393 days for seed formation to occur). No seeds were produced in uncut plots. At the end of 9 years, mortality rates of bellyache bush plants initially planted averaged ≥73% for treatments with some pasture compared with 55% under the no-pasture treatment. The percentage of herbaceous plant basal cover in uncut plots was increased 5-fold after 9 years, much greater than the average 2% increase recorded across the low, medium, and high pasture treatments. The number of herbaceous species in uncut plots remained largely unchanged, whereas there was an average reduction of 46% in the cut pasture treatments. Buffel grass remained the species with the greatest basal cover across all cut pasture treatments, followed by sabi grass (Urochloa mosambicensis (Hack.) Dandy) and then red Natal grass (Melinis repens (Willd.) Ziska). These results suggest that grazing strategies that maintain a healthy and competitive pasture layer may contribute to reducing the rate of spread of bellyache bush and complement traditional control techniques such as the use of herbicides.

Bellyache bush (Jatropha gossypifolia L.) is an invasive shrub that adversely impacts agricultural and natural systems of northern Australia. While several techniques are available to control bellyache bush, depletion of soil seed banks is central to its management. A 10-year study determined the persistence of intact and ant-discarded bellyache bush seeds buried in shade cloth packets at six depths (ranging from 0 to 40 cm) under both natural rainfall and rainfall-excluded conditions. A second study monitored changes in seedling emergence over time, to provide an indication of the natural rate of seed bank depletion at two sites (rocky and heavy clay) following the physical removal of all bellyache bush plants. Persistence of seed in the burial trial varied depending on seed type, rainfall conditions and burial depth. No viable seeds of bellyache bush remained after 72 months irrespective of seed type under natural rainfall conditions. When rainfall was excluded seeds persisted for much longer, with a small portion (0.4%) of ant-discarded seeds still viable after 120 months. Seed persistence was prolonged (>96 months to decline to

Bellyache bush (Jatropha gossypiifolia L.) is an invasive weed that has the potential to greatly reduce biodiversity and pasture productivity in northern Australia’s rangelands. This paper reports an approach to develop best practice options for controlling medium to dense infestations of bellyache bush using combinations of control methods. The efficacy of five single treatments including foliar spraying, slashing, stick raking, burning and do nothing (control) were compared against 15 combinations of these treatments over 4 successive years. Treatments were evaluated using several attributes, including plant mortality, changes in population demographics, seedling recruitment, pasture yield and cost of treatment. Foliar spraying once each year for 4 years proved the most cost-effective control strategy, with no bellyache bush plants recorded at the end of the study. Single applications of slashing, stick raking and to a lesser extent burning, when followed up with foliar spraying also led to significantly reduced densities of bellyache bush and changed the population from a growing one to a declining one. Total experimental cost estimates over 4 successive years for treatments where burning, stick raking, foliar spraying, and slashing were followed with foliar spraying were AU$408, AU$584, AU$802 and AU$789 ha–1, respectively. Maximum pasture yield of 5.4 t ha–1 occurred with repeated foliar spraying. This study recommends that treatment combinations using either foliar spraying alone or as a follow up with slashing, stick raking or burning are best practice options following consideration of the level of control, changes in pasture yield and cost effectiveness.

Understanding the dispersal of weed species is important for the development of effective control strategies. In this study, a series of experiments was conducted to clarify the role that meat ants (Iridomyrmex spadius) play in dispersing bellyache bush (Jatropha gossypiifolia), an exotic shrub currently invading the rangelands of northern Australia. The nutrient composition of food [lipids (fatty acids), fat and soluble carbohydrates] provided by bellyache bush seed components [caruncle, exotegmen and seed (without caruncle and exotegmen)] was identified. Seed components were rich in lipids, particularly palmitic, oleic, stearic, linoleic and eicosenic acids. Oleic and palmitic were most abundant in the caruncle (30% each), linoleic in the seed (61%) and palmitic in the exotegmen (36%). Over all seed components, fat concentration was relatively high (6.3%) compared with soluble carbohydrates (2.3%). The impact of feeding was then determined by comparing germination and viability of intact, non-carunculate (caruncle manually removed) and ant-discarded bellyache bush seeds. Feeding by meat ants significantly increased seed germinability, whilst having no adverse affects on viability. The quantity of seeds dispersed and the seasonal pattern of dispersal was recorded by collecting seed from the middens of randomly selected meat ant nests on a monthly basis. On average, 12 330 ± 603 seeds were retrieved from the middens of individual meat ant nests over 12 months, with highest numbers recorded between February and June (>1200 seeds/ant nest). The effect of this dispersal was determined through comparisons of plant densities within core infestations of bellyache bush, meat ant nest middens and pastures located directly adjacent to core infestations and that were being invaded primarily through localised ballistic dispersal. The density of bellyache bush plants growing from the seed reserves within middens averaged 79 plants/m2, just 18% less than that within core infestations. Seedling survival (1 year) and growth within core infestation and meat ant sites was also quantified. The middens of meat ant nests provided an environment conducive to higher seedling survival and faster growth rates than occurred within core infestations. Mutualistic interaction between bellyache bush and meat ants is likely to build local ‘infestation pressure’ that may be conducive to range extension in years of exceptionally wet seasons. Management of seed dispersal by meat ants may reduce that risk.

This study compared the efficacy of first and second fires applied during the early (August–September) and late dry season (October–November) on mortality of riparian (climbing) and subriparian (freestanding) infestations of rubber vine (Cryptostegia grandiflora R. Br.). The impact of fire treatments on germinable seed banks of monocotyledonous and dicotyledonous species was also determined. Individually, fire season, habitat type and number of fires significantly affected mortality of rubber vine plants. Late-season fires promoted higher mortality of rubber vine (96%) than early season fires (77%), with rubber vine in subriparian habitats more susceptible (90% mortality) than that growing in riparian areas (68% mortality). On average, fire mortality increased from 32% after the first fire up to 86% following 2 fires. Sensitivity of juvenile, mature, and old rubber vine plants to fire was in the order of mature>juvenile>old. Early fires significantly reduced seed banks of monocotyledonous plants, particularly in riparian habitats. Late fires significantly reduced seed banks of both dicotyledonous and monocotyledonous plants. No rubber vine seeds were detected in the germinable seed bank of either burnt or unburnt plots.

If treatments imposed to control exotic plants also have a deleterious impact on their residual seed bank, the duration and extent of follow-up control may be reduced. Fire is one such technique that has this ability, particularly if seeds are located on or close to the soil surface. Three studies were undertaken in a riparian habitat in the dry tropics of northern Queensland to quantify the effects of spring burning on the seed bank of the exotic weed bellyache bush (Jatropha gossypiifolia). The first determined the distribution of seeds within the vertical profile of the trial site. The other 2 studies were experiments that quantified the effects of fire on germination and viability of both dispersed bellyache bush seeds and seeds held in mature capsules. Dispersed seeds of 2 types (intact and ant-discarded) were placed at the following 6 positions in the vertical profile of bellyache bush infestations: 0.5, 1, 2 and 4 cm depth below ground, on bare ground, and below fuel. Seeds held in capsules were located at random on the crown of bellyache bush plants. For both experiments, comparisons of burnt plots were made with unburnt controls. Fire was imposed in spring (September); the season in which burning for weed control in northern Queensland generally occurs. While maximum fire temperatures averaged 590 ± 46°C, the temperatures that seeds or capsules were exposed to depended on their location within the vertical profile, with temperatures decreasing in the following order: below fuel > crown of bellyache bush > 1 cm > 0.5 cm > bare ground > 2 cm > 4 cm below ground. There were negative correlations between seed germination and peak fire temperature and between viability and peak fire temperature. Seed viability was nil for seed under fuel but >80% for seeds placed on bare ground or ≥2 cm below ground. Fire reduced germination and viability of seeds held in capsules by 31 and 35%, respectively, when compared with unburnt seeds. While ant-discarded seeds generally had a higher germinability than intact seeds, they were more susceptible to fire. This may be attributed to loss of the external protective barrier of the seed coat (exotegmen) caused by the feeding of ants. Bellyache bush seeds were recorded across all soil depths, reaching a peak of 3.8 million seeds per hectare at 1–5 cm soil depth. These results suggest that while bellyache bush seeds are susceptible to fire, many are buried beyond the reach of lethal temperatures. Therefore, viable seeds will be available for post-fire recruitment and other measures, such as chemical control, may need to be employed in conjunction with burning.

Studies were undertaken at 2 sites in northern Queensland (Wrotham Park and Ten Mile Creek) to quantify the effects of fire on germination and viability of both loose rubber vine seeds and seeds held within pods. Seed lots were placed at a range of positions within the vertical profile of rubber vine infestations; 0.5 cm below ground (loose seeds only), on bare ground, below fuel, above fuel and on crowns of free-standing rubber vine. The influence of seed age (0 and 1 year old) and seed plumage (with and without plumes) was also examined for loosely held seeds. Late season fires were imposed at both Wrotham Park and Ten Mile Creek, with an additional early season fire undertaken at Ten Mile Creek to enable comparison between seasonal fires. Maximum fire temperatures reached during burning averaged 853, 827 and 871˚C for Wrotham Park, Ten Mile Creek early and Ten Mile Creek late season fires, respectively. The temperatures that seeds were exposed to depended on their location within the vertical profile, with temperatures above fuel > below fuel > on rubber vine crown > bare ground > 0.5 cm below ground. Seed viability after burning ranged from 0 to 100%, with seeds generally most affected if located above or below the fuel, and least affected if buried below ground. The age of seed and the presence of seed plumes interacted with seed placement so that seed germination and viability of loose seeds were affected. Some seeds remained viable, but were inhibited from germinating, particularly if located 0.5 cm below ground. Positive, non-linear relationships occurred between seed mortality and maximum temperature and between seed mortality and the duration of exposure. These results suggest that fire may inhibit germination and kill a substantial proportion of rubber vine seeds.

Phenolic content, germination percentage, and host preference were compared among 11 seed populations of witchweed (Striga hermonthica(Del.) Benth. # STRHE). Significant differences in germination percentage occurred among the seed populations after preconditioning by their exposure to different durations of warm-moist conditions. Greatest percentage of seed germinated after 8 days of preconditioning. Preconditioning for more than 16 days reduced percentage germination. The host specificity range of witchweed seed populations that parasitized pearl millet [Pennisetum americanum(L.) K. Schum] was broader than were those seed populations that attacked sorghum [Sorghum bicolor(L.) Moench.]. Results of thin-layer chromatography indicated large variations in phenolic content among the witchweed seed populations.

The dispersion of ethylene in the soil after appli- cation from a backpack ethylene applicator (BPEA) was determined using a bioassay of witchweed (Striga asiatica (L.) Kuntze #4 STRLU) seed germination. Witchweed seed were buried in packets in ridged seedbeds under field condi- tions. Ethylene dispersion was better when injected into the furrow than into the ridge top. Injections 30 cm deep were more effective in causing seed germination than in- jections 15 cm deep. Seed germination induced by ethylene injection into the furrow 30 cm deep was 13% more than that obtained from the ridge top injection 15 cm deep. Seed germination as high as 90% was obtained at soil depths to 75 cm at a horizontal distance of 100 cm from the point of ethylene injection. Ethylene diffused up to 300 cm hori- zontally and to 75 cm vertically to cause at least 45% seed germination in a field of Lakeland soil (Typic Quartzisamn- ment). The BPEA was found to be an effective application tool in witchweed control operations. Additional index words. Germination stimulants, parasitic weed, Striga asiatica, witchweed, STRLU.

Increasing plant density of grain sorghum [Sorghum bicolor (L.) Moench.] from 57100 to 114300 plants/ha had no significant effect on grain yields, but significantly increased Striga hermonthica (Del.) ‘Benth’ # STRHE shoot yields by 49% in S. hermonthica infested soil of the Sudan. Addition of urea at 43, 86, and 172 kg N/ha increased grain yields by 14, 25, and 45%, respectively, above the control and S. hermonthica shoot yields were reduced by 14, 21, and 35%, respectively, below the control. Although average grain yields in 1981 were 77% less than in 1980, S. hermonthica shoot yields were significantly increased by about sixfold. Grain sorghum yields of pure stands were not consistent over both years. In 1980, the cultivar ‘Ariana’ outyielded ‘Tetron’, but in 1981 the reverse occurred. However, in both years, the 50:50 mixture of both cultivars gave intermediate grain yields. While pure stands of Tetron’ sustained the lowest infestations of S. hermonthica and ‘Ariana’ had the greatest, the mixture had intermediate infestations which were on average 31% less than those of ‘Ariana’. Thus, the results suggest that the cultivation of equiproportional mixtures of relatively resistant and susceptible grain sorghum varieties, coupled with the use of high rates of nitrogenous fertilizer and wider inter- and intra-row spacing, may be a better cultural practice under dense striga infestations.

Leaf temperatures (Ti) of the parasitic plant Striga hermonthica are substantially below those of the air (TJ, [Ta — T,] reaching 7 °C at Ta = 40 °C. This results from high rates of transpiration and the consequent evaporative cooling of the leaf. Application of an antitranspirant, which mechanically impedes foliar loss of water vapour, reduced transpiration and stomatal conductance by 40% and 57%, respectively, and reduced \Ta— TJ to 2 °C at 7*a = 40 °C. The temperature sensitivity of photosynthesis in the host-parasite association differed, the optima (Topt) being 37-2 and 40-1 °C for S. hermonthica and sorghum, respectively. Once ropt had been exceeded in S. hermonthica net photosynthesis declined rapidly, reaching the lethal limit (Tmax) at 42-6 °C. S. hermonthica is particularly sensitive to high temperatures and antitranspirant-induced overheating leads to blackening and shrivelling of the leaf after as little as 4 h at Ta = 40 °C. Application of an antitranspirant under field conditions in the Sudan at 7; = 40 °C resulted in 28% and 67% reductions in transpiration and stomatal conductance, together with a 5 °C increase in T1 and subsequent leaf death. In addition to these short-term physiological responses, antitranspirant spraying of the parasite increased the grain and straw yield of the crop by factors of 3-4 and 2-6, respectively. Antitranspirants may have potential use as a method of controlling Striga in the field.

SUMMARYPreliminary evidence is presented in this study for the existence of (1) intraspecific physiological variants of Striga hermonthica, with Gabob population as the most responsive strain to a range of Sorghum bicolor cultivars, and (2) a local Sudanese sorghum cultivar called Tetron with broad-spectrum resistance to striga. Thus Tetron may be recommended as another standard resistant cultivar with reference to Framida of East Africa.

Little is known about forage sorghum [ (L.) Moench] and forage sorghum-sudangrass [ × ] hybrid production in soil infested with witchweed [ (Del.) Benth]. The objective of the present study was to evaluate the immediate effects (first harvest) and aftereffects (second and third harvest) of two cutting heights (0.1 and 0.3 m) and five N treatments (0,60 + 60,120,120 + 120, and 240 kg N ha−) on witchweed management in forage sorghum (cv. Abu Sabeen) and forage sorghum-sudangrass (cv. Pioneer 988) hybrid. Field studies were conducted at Shambat, Sudan, in 1984 and 1985 on a heavy alluvial clay soil (montmorillonitic, isohyperthermic Vertic Camborthid) heavily infested with witchweed. Pioneer 988 was more tolerant of witchweed than Abu Sabeen at first harvest, but less tolerant of witchweed at later harvests. The shorter cutting height resulted indirectly in a larger number of tillers (26%) and a higher forage greenchop yield (37%) at first harvest. Nitrogen increased forage tiller density approximately twofold and forage greenchop yield by approximately ten-fold at the first harvest. There was a concurrent, approximately fivefold reduction in witchweed shoot weight. At second harvest, forage tiller density, forage greenchop yield, and witchweed shoot weight were increased with N. Both forage sorghum and witchweed were dead in the zero N plots at third harvest. No differences were detected between single and split applications of N, or between low (120 kg N ha−) and high (240 g N ha−) rates of N. Forage tiller density, forage greenchop yield and witchweed shoot weight were reduced at the second and third harvests compared to the first harvest.

Witchweed (Striga asiaticaLour. ♯4STRLU) seed remained viable for 6 yr under open-shelf laboratory conditions and for 14 yr when deep buried under field conditions. Seed kept in normal atmosphere of the laboratory for < 1 through 6 yr had the following germination percentages: 2, 95, 95, 94, 83, 38, and 7. Newly harvested seed will not germinate because of postmaturity ripening requirements. There were no viable seed after 7 yr of shelf storage in the laboratory. Seed buried in the field gave 60 to 90% germination the first season after production. Annual recovery and testing showed a gradual reduction in viability to 10% at year 14 for the deepest buried seed; shallower buried seed expired sooner. No germination occurred thereafter.

SUMMARY Four experiments were conducted to compare the yield, and stability of yield of oats and barley grown separately and in an equiproportional mixture (w/w). The cultivars used were Maris Mink and Karin, chosen to synchronize, as far as possible, their maturity dates. In general, the mixtures marginally overyielded the more productive species but not significantly so. When barley was sown at half field density, grain yield did not drop. In contrast, the yield of oats was lower when plots were sown at lower density. Barley was generally more unstable in its yield than oats, which it outyielded on good sites but underyielded on poor sites. Mixtures were much more stable than barley, and almost as stable as oats, but were never the most stable type of plot.

Seed size and weight significantly affected witchweed [Striga asiatica(L.) Kuntze =S. luteaLour. ♯4STRLU] seed germination, witchweed emergence, and host-parasitization. Witchweed seed were divided into four groups for testing; large (> 149 microns in diameter) and heavy (specific gravity greater than one) (LH), large and light (specific gravity less than one) (LL), small (< 125 microns in diameter) and heavy (SH), and small and light (SL). Germination percentages were 87, 84, 58, and 26 for LH, LL, SH, and SL, respectively. Corn (Zea maysL.) was planted in pots containing soil infested with these four seed categories. The total number of witchweed plants (submerged plus emerged) per pot containing LH, LL, SH, and SL witchweed seed was 65, 12, 0, and 0, respectively, and the number of witchweed plants that emerged was 19, 4, 0, and 0 per pot. There were no significant differences in shoot weights of corn grown in the pots either between kinds of witchweed seed infesting the soil or between plants grown in infested soil and plants grown in soil that did not contain witchweed seed. Roots of corn plants grown in soil infested with LH seed were significantly heavier than those grown in soil infested with any other seed class and heavier than those from soil that did not contain witchweed seed. No other seed class significantly affected root weight.

SUMMARYThe effects of sowing pattern and seed rate on the green-chop yield of forage sorghum-sudangrass hybrid (cv. Pioneer 988) and forage sorghum (cv. Abu Sabeen) were investigated. In general, more green-chop yield was obtained from seed of both cultivars drilled in regular rows, double rows and triple rows than when traditionally sown as broadcast seed. Yields were greater when plants were sown at 70 kg seed ha−1 than when traditionally sown at 35 kg seed ha−1. These yield advantages were sustained through three successional harvests. Pioneer 988 generally yielded more than Abu Sabeen, especially when sown in January. Both cultivars gave minimum yield when sown in April, July or October.

Seeds of grain sorghum (Sorgham bicolor (L.) Moench) varieties were irradiated with gamma rays from a 60Co source. The sorghum variety ‘Tetron’ gave maximum resistance to witchweed (Striga hermontica (Del.) Bent). In both laboratory and glasshouse experiments, sorghum seedlings obtained from seeds irradiated at 5, 10, 15, 20 and 30 krad reduced relative to control witchweed seed germination and attachment by 48, 64, 76, 86 and 91%, respectively, and by 53, 39, 46, 89%, respectively. In addition, the root yields of 6-week-old sorghum plants were reduced by 18, 33, 38, 51 and 75%, respectively, below the control while shoot yields were increased by 151, 72, 46, 30 and 6% respectively, above the control. In the field experiment, the irradiation treatments of 5, 10, 15 and 20 krad reduced both witchweed shoot yields and sorghum grain yields by 57, 78, 54 and 48%, respectively, and 15, 47, 52 and 60%, respectively, below the control. The significant reduction in sorghum root yield may account for the low sorghum grain yields and the low witchweed infestations associated with the low stimulant production of the sorghum roots.

SUMMARYThe parasitic weed striga had significantly greater effects on sorghum yield components than non-parasitic weeds. Grain reduction caused by striga amounted to 65% compared with 32% reduction caused by non-parasitic weeds. Comparable reductions were also manifest in straw yield, grains/head and plant height of sorghum. Early removal of striga (a week after its emergence), and thereafter at regular weekly intervals, may help to improve crop yields.

In two species mixtures over all nutrient levels and harvests, mixtures of cocksfoot and timothy invariably gave relative yield total (RYT) values well above 10 thus implying a synergistic relationship, while mixtures of cocksfoot and perennial rye grass (PRO) consistently gave RYT values well below 1-0 thus indicating a mutually antagonistic relationship. However, mixtures of PRO and timothy generally gave RYT values approaching 10. Similarly mixtures of cocksfoot and timothy overyielded their respective highest yielding pure stands and mixtures of cocksfoot and PRG underyielded their respective lowest yielding pure stands. PRG and timothy generally gave intermediate yields between those of their highest and lowest pure stands. Relative yield values over all nutrient levels and harvests revealed that PRG is more aggressive than cocksfoot, which in turn is more aggressive than timothy. In a three species mixture this trend in order of aggressiveness was also maintained. The yields of the three species mixtures invariably exceeded those of their highest yielding pure stand. In both experiments pure stands of cocksfoot gave the highest yields and timothy the lowest. Nitrogen fertilization significantly affected herbage yield maximizing it at high nutrient level. There were also significant differences in yield between harvests over all nutrient levels. However, the particular combination of species in a mixture affected the mixture behaviour and yield, though generally the behaviour of a component in a mixture was not predictable from a knowledge of its behaviour in pure stands. The present findings also suggest that the hypothesis that mixtures yield more than pure stands is untenable as a general statement about all mixtures.

Ethylene gas stimulated seed germination of the pearl millet [Pennisetum americanum(L.) K. Schum] strain ofStriga hermonthica(Del.) Benth. # STRHE) in two soil types in the North Kordofan Region of Sudan. Soils were the noncracking clay pediplain or ‘Gardud’, in Arabic, and the stabilized sand locally called ‘Qoz’. Ethylene at 1.5 kg ai/ha induced greater seed germination (suicidal seed germination) in the Gardud soil than in the Qoz soil. However, ethylene at 3.0 kg ai/ha was better on Qoz than Gardud soil. The effect of injection depth of ethylene was dependent on soil type. Ethylene was more effective when injected at a 15-cm depth in the Gardud soil and at a 30-cm depth in the Qoz soil. The viable population ofS. hermonthicaseed was reduced by suicidal seed germination at soil depth to 60 cm and to a horizontal distance of 25 cm from the point of ethylene injection. Ethylene-induced germination reduced seed population by 67 and 34% in Gardud and Qoz soils, respectively. A single injection of ethylene diffused up to 300 cm horizontally and up to 60 cm vertically and caused 12.6 and 8.5% reduction in viable 5.hermonthicaseed in Gardud and Qoz soils, respectively. The potential of ethylene gas as an effective tool in striga control operations in a pediplain and stabilized sand soil such as is found in N. Kordofan is dependent upon the availability of the gas and the injection apparatus and the economics of its application.

SUMMARYNitrogen-atrazine combinations were significantly more effective in stimulating higher yield components of sorghum and depressing yield components of striga than sole treatments of either nitrogen or atrazine. Over all treatments nitrophoska at 129 kg N/ha in combination with atrazine at 1.75 kg/ha gave 224, 180, 155 and 151% increases in yield components over the control for head, straw, 1000 grain weights and plant height of sorghum, and a decrease in 83, 88 and 90% of striga density at 8, 12, and 16 weeks from sowing the sorghum. The synergistic effect of nitrogen-atrazine is demonstrated.

Experiments were performed to evaluate the effect of different sowing patterns on the yield of mixtures of cocksfoot and timothy, and of oats and barley. The grass mixtures significantly overyielded the pure stands. Differences in yield between the patterns were less than 9% in cereals and less than 3% in grasses. None of these differences was statistically significant, though agronomically, if sustained, might represent an attractive increase in revenue. Oats and cocksfoot became the dominant components of their mixtures, and each performed best in the “most mixed” stands. Sowing in alternate rows produced the greatest stability of yield in both cereal mixtures and mixtures of forage grasses.

The effects of various concentrations of the herbicide 2,4-D were investigated on two vegetative forms of water hyacinth including juvenile and mature forms. The results obtained suggest that efficient chemical control of water hyacinth may be best applied at the early stages of vegetative development of the water hyacinth, i.e. at the juvenile phase. This would require the use of 2,4-D at a concentration of 60 000 ppm (2.2 kg/ ha) which equals to half the rate of 2,4-D normally used for control of water hyacinth in the Sudan. The lower rate of 2,4-D not only means a considerable reduction in quantity and cost of the consumed herbicide but also a substantial minimisation in the level at which pollution may build up in the White Nile of the Sudan.