Search

Atmospheric carbon‐dioxide concentration ([CO2]) is increasing rapidly, but its interactions with nitrogen (N) and phosphorus (P) fertiliser on wheat grain quality are not well understood. We investigated the effects of ambient CO2 (aCO2; ∼410 ppm) and elevated CO2 (eCO2; 760 ppm) on crop harvest index (CHI), nutrient harvest index (NuHI), shoot macro‐nutrient content and grain macro‐nutrient concentration of wheat grown under two contrasting amounts of N (0.5 and 6 mol m−3 NO3− N) and P (10 and 250 mmol P m−3) fertiliser supply (low and optimum, respectively). Our results highlighted interactions between [CO2] and N and P fertiliser supply for the shoot biomass at anthesis and straw biomass at harvest maturity. This was because biomass yield did not respond to CO2 level when fertiliser was deficient. However, shoot and straw yield increased (10.0–‐34.0%) with increasing [CO2] at optimum fertiliser rates. Across experiments, grain yield increased (15.6%) with increasing [CO2], which resulted in grain nutrient concentration decreasing (3.0–‐13.0%) with increasing [CO2]. This was attributed to nutrient 'dilution' due to increased carbohydrate content in the grain. Overall, fertiliser supply impacted crop responses more than CO2 treatments, and the impact was greater under N than P deficiency. This was reflected through conservative values for CHI, thousand grain weight and NuHIs suggesting plants allocated biomass and nutrients at similar rates for vegetative and reproductive organs independent of [CO2]. [ABSTRACT FROM AUTHOR]

Crop growth is related to radiation-use efficiency ( RUE), which is influenced by the nitrogen ( N) status of the crop, expressed at canopy level as specific leaf N ( SLN) or at plant level as N nutrition index ( NNI). To determine the mechanisms through which N affects dry-matter ( DM) production of forage kale, results from two experiments ( N treatment range 0-500 kg ha−1) were analysed for fractional radiation interception ( RI), accumulated radiation ( Racc), RUE, N uptake, critical N concentration ( Nc), NNI and SLN. The measured variables ( DM, RI and SLN) and the calculated variables ( NNI, Racc and RUE) increased with N supply. RUE increased from 0·74 and 0·89 g MJ−1 IPAR for the control treatments to 1·50 and 1·95 g MJ−1 IPAR under adequate N and water in both experiments. This represented an increase in RUE of 52-146% for the range of N treatments used in both experiments, whilst Racc increased by 9-17%, compared with the control treatments. Subsequently, the total DM yield of kale increased from 6·7 and 8 t DM ha−1 for the control treatments to ≥ 19 t DM ha−1 when ≥150 kg N ha−1 was applied. The DM yields for the 500 kg N ha−1 treatments were 25·5 and 27·6 t DM ha−1 for the two experiments. RUE increased linearly with SLN, at an average rate of 0·38 g DM MJ−1 IPAR per each additional 1 g N m−2 leaf until a maximum RUE of 1·90 g MJ−1 IPAR was reached in both experiments. There were no changes in RUE with SLN of > 2·6 g m−2 and NNI >1, implying luxury N uptake. RUE was the most dominant driver of forage kale DM yield increases in response to SLN and NNI. [ABSTRACT FROM AUTHOR]

Nitrogen fertilizer is applied to supplement soil nitrogen supply to maximize forage brassica crop dry-matter production. However, nitrogen fertilizer applications in excess of that required to maximize growth result in potentially toxic nitrate-nitrogen ( NO3-N) concentrations in grazeable plant tissues. Three experiments, two for forage kale at Lincoln ( Canterbury) and one for forage rape at Hastings ( Hawke's Bay) in New Zealand were grown under different rates of nitrogen (0-500 kg N ha−1) to determine the effect of different rates of nitrogen on NO3-N content of different plant parts of the crops. One of the kale experiments was grown with either full irrigation or no rain and no irrigation over summer, hereafter referred to as summer drought. The NO3-N concentration on a whole plant (weighted average) basis increased from 0·1 mg g−1 dry matter for the control plots to 2·30 mg g−1 for the 500 kg N ha−1 plots for forage kale. It increased from 0·99 for the control plots to 3·37 mg g−1 for the 200 kg N ha−1 plots for forage rape crops. However, NO3-N concentration increased with N supply under the summer-drought plots from an average of 0·33 mg g−1 when ≤120 kg N ha−1 was applied to 2·30 mg g−1 for the 240 kg N ha−1 treatments but was unaffected by N supply under irrigation. The NO3-N concentrations were higher in the stems and the petiole (which included the midrib of the leaf) than leaves in all three experiments. The NO3-N concentration was highest at the bottom of the kale stem and decreased towards the top. We recommend N application rates based on soil tests results, and for conditions similar to the current studies up to 300 kg N ha−1 under irrigation and adjusted lower N rates for regions prone to dry summers. [ABSTRACT FROM AUTHOR]

Accuracy of determining radiation interception, and hence radiation use efficiency, depends on the method of measuring photosynthetically active radiation intercepted. Methods vary, from expensive instruments such as Sunfleck ceptometers to simple methods such as digital photography. However, before universal use of digital photography there is need to determine its reliability and compare it with conventional, but expensive, methods. In a series of experiments at Lincoln, New Zealand, canopy development for barley, wheat, white clover and four forage brassica species was determined using both digital photographs and Sunfleck ceptometer. Values obtained were used to calculate conversion coefficient (K f/K i) ratios between the two methods. Digital photographs were taken at 45° and 90° for barley, wheat and white clover and at only 90° for brassicas. There was an interaction of effects of crop and cultivar for the cereal crops. Barley closed canopies earlier than wheat, and 'Emir' barley and 'Stettler' wheat had consistently higher canopy cover than 'Golden Promise' and ' HY459′, respectively. Canopy cover was consistently larger at 45° than 90° for cereals. However, for white clover, the angle of digital photography was not important. There was also an interaction between effects of species and method of determining canopy cover for brassicas. Photographs gave higher cover values than ceptometer for forage rape and turnip, but the relationship was variable for forage kale and swede. K f/K i ratios of 1.0-1.10 for cereals, white clover and forage rape and turnip show that digital photographs can be used to estimated radiation interception, in place of Sunfleck ceptometer, for these crops. [ABSTRACT FROM AUTHOR]

Establishing the radiation-use efficiency ( RUE) of forage brassica crops will aid our understanding of their photosynthetic performance. The concept of RUE has been developed for cereals and legumes, but there is limited information for forage brassica crops. Three experiments defining the influence of different sowing dates on 'Gruner' kale ( Brassica oleracea acephala L.) dry matter production were conducted at Hastings (Hawkes Bay) and Lincoln (Canterbury) in New Zealand between 2002 and 2009. These trials were also evaluated for radiation interception and RUE. Delayed sowing increased RUE in two out of three experiments across sites: from 1·93 g MJ−1 photosynthetically active radiation ( PAR) for December-sown crops to 2·72 g MJ−1 PAR ( P < 0·001) for January-sown crops at Hastings and from 1·50 for September-sown crops to 2·00 g MJ−1 PAR ( P < 0·001) for November-sown crops at Lincoln. The different sowing dates and years of experimentation provided a range of mean temperatures (from 13 to 16°C) during the vegetative period. Across years and sowing dates, RUE was strongly correlated with mean temperature (R2 = 0·81) and sowing date (R2 = 0·64), but weakly correlated with season length (R2 = 0·11) and dry matter (R2 = 0·002). There was also a strong correlation (R2 = 0·83) between sowing date and mean temperature. The increase in RUE with delayed sowing was therefore mainly attributed to increased mean temperatures. Radiation-use efficiency increased at about 0·41 g MJ−1 for each 1°C increase from 13 to 16°C. [ABSTRACT FROM AUTHOR]

Defining the critical nitrogen concentration (Nc; g N kg−1) for maximum growth of forage brassicas will aid in the fertilizer management of these crops. Typically, the Nc value decreases with increasing crop biomass. In this paper, we used a nitrogen (N) response experiment with kale ( Brassica oleracea) to define a critical N dilution (Nc = 55·3 × biomass−0·47). However, at biomass <3·4 t ha−1, a constant NC of 31·2 g N kg−1 was found. This N dilution curve compared favourably with published data sets for a range of forage brassicas but was substantially different from the established N dilution curve for oilseed rape ( Brassica napus). This study also found a strong relationship ( R2 = 0·81) between the nitrogen nutrition index (NNI) and the NO3 content of forage brassicas from a range of data sets. The NNI is the actual N concentration of the shoot as a ratio of the Nc from the established curve. The relationship between NNI and NO3 contents was significantly different between leafy forage brassica crops and root forage brassicas. For each 0·1 increase in NNI, the proportion of total N that was in the form of NO3 increased by 2·7% for leaf/stem brassicas and 0·60% for root crop brassicas. The critical dilution curve defined in this study can be used to manage fertilizer N in forage brassica crops, so that growth can be maximized but the risk of high NO3 concentrations in the forage can be minimized. [ABSTRACT FROM AUTHOR]