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The leukotoxin of Pasteurella (Mannheimia) haemolytica is believed to play a significant role in pathogenesis, causing cell lysis and apoptosis that lead to the lung pathology characteristic of bovine shipping fever. Using a system for Cre-lox recombination, a nonpolar mutation within the lktC transacylase gene of the leukotoxin operon was created. The lktC locus was insertionally inactivated using a loxP-aph3-loxP cassette, and then the aph3 marker was excised from the chromosome by Cre recombinase expressed from a P. haemolytica plasmid. The resulting lktC strain (SH2099) secretes inactive leukotoxin and carries no known antibiotic resistance genes. Strain SH2099 was tested for virulence in a calf challenge model. We inoculated 3 x 10(8) or 3 x 10(9) CFU of wild-type or mutant bacteria into the lungs of healthy, colostrum-deprived calves via transthoracic injection. Animals were observed for clinical signs and for nasal colonization for 4 days, after which they were euthanized and necropsied. The lower inoculum (3 x 10(8) CFU) caused significantly fewer deaths and allowed lung pathology to be scored and compared, while the 3 x 10(9) CFU dose of either the wild-type or mutant was lethal to >/=50% of the calves. The estimated 50% lethal dose of SH2099 was four times higher than that of the wild-type strain. Lung lesion scores were reduced twofold in animals inoculated with the mutant, while clinical scores were nearly equivalent for both strains. The wild-type and mutant strains were equally capable of colonizing the upper respiratory tracts of the calves. In this study, the P. haemolytica lktC mutant was shown to be less virulent than the parent strain.

A new system was developed and tested for efficient use of irrigation and rainfall for furrow irrigation of grain sorghum (Sorghum bicolorL. Moench). The design uses a limited water supply to irrigate a larger area than could be fully irrigated and thus reduces the area in dryland sorghum where a farmer produces both. A 600‐m long field was divided into three water management sections. The upper half of the field was managed as fully irrigated. The next one‐fourth was a “tail‐water‐runoff” section that utilized furrow runoff from the fully irrigated section. The lower one‐fourth was designated a “dryland” section, retaining and utilizing any runoff generated either by irrigation or rainfall from the wetter, fully irrigated, or tailwater‐runoff sections. Furrow dams were placed about every 3 to 4 m throughout the length of the field after planting. However, these were washed out by the irrigation water over the distance the water advanced down the furrows. The remaining furrow dams on the lower part of the field stopped rainfall runoff and prevented any water from leaving the field. As the profile soil water was depleted on the drier lower field sections, shrinkage cracks developed that helped to absorb rainfall and prevent rainfall runoff. Seeding rates were reduced down the field from 6 kg/ha on the fully irrigated section to 3 kg/ha on the tailwater section, to 1.5 kg/ha on the dryland section. Nitrogen applications were also reduced on the tailwater and dryland sections to correspond with anticipated yield levels. The three irrigation treatments were 5‐cm water applied to every furrow; 3.75‐cm water applied to alternate furrows; and 2.5‐cm water applied to every third furrow between crop rows in a two‐row planted, and one row left out, skip‐row system. These treatments were compared with conventional furrow irrigation without dams involving tail‐water runoff. Five irrigations were applied at 14‐day intervals, regardless of seasonal rainfall. The extent that the field was irrigated depended on the rainfall. The fixed 14‐day irrigation schedule and 12‐hour sets reduced management requirements and facilitated the potential for automation of graded furrow water application, without the farmer having to be concerned about tailwater runoff from the end of the field. For 1979, a year with near average rainfall, the system successfully prevented runoff from both irrigation and rainfall.

Major variability occurs in yield response of irrigated winter wheat (Triticum aestivumL. em Thell.) to water deficits and irrigation in the Southern High Plains. Evaluation of plant growth and development and yield components are needed to better understand water relations and to develop and test growth and yield models. The study was conducted to evaluate the effects of three planting dates, four to six water deficit treatments during stages of plant development, and two short wheats, ‘TAM W‐101’ and ‘Vona,’ differing in yield components. Planting dates were early (15 September), normal (12 October), and late (7 November). Water deficits were established by withholding irrigation and were terminated at selected plant stages by applying irrigation to level border plots. The study was conducted on a slowly permeable Torrertic Paleustoll (Pullman clay loam). We evaluated development stages, tillering, plant water potential, grain and dry matter yields, and yield components of heads per m2spikelets per head, grain per spikelet, grain weight and number per m2. Early to normal planting, as compared with late planting (i) advanced the dates of phenological development, (ii) lengthened the time in the vegetative period, (iii) lengthened the time from floral initiation to inflorescence, (iv) increased tillering during vegetative growth and tiller senescence during jointing, and (v) increased dry matter and grain yields. Plant water deficits reduced yields by increasing tiller and stem senescence, which reduced harvested heads, and by reducing grain numbers (spikelets per head and grain per spikelet). Vona produced more secondary tillers and grains per head, but tiller‐stem senescence and smaller grain size resulted in harvested head numbers and grain yield similar to those of TAM W‐101. High yields (5,580 kg/ha) were associated with adequate fall‐tillering and irrigation during spring vegetative growth that minimized stress‐associated tiller‐stem senescence and reduction in spikelets per head. TAM W‐101 was more resistant than Vona to severe desiccation and tillerstem senescence under severe water deficits. A fall irrigation applied before winter dormancy delayed the need for a spring irrigation until jointing. Irrigation just before a period of above average rainfall did not affect yields. Yields comparisons of TAM W‐101 and Vona indicate that similar high yields can be produced by varieties having large differences in seed size and number per m2, suggesting that specific yield components may not be closely correlated with high yield potential.

The dependence of the rate constant for free-radical decay on pressure and concentration of chemical crosslinks has been studied in irradiated copolymer of methyl methacrylate and ethylene dimethacrylate. A decrease in the sensitivity of the rate constant for decay to pressure changes with the increasing degree of crosslinking has been found. The effect of molecular motions of backbone on the transport of a radical center in solid polymer is shown.

A Limited Irrigation‐Dryland (LID) farming system for the conjunctive use of rainfall and limited irrigation of graded furrows was developed and field tested. The unique feature of the LID system is that it actually adjusts, during the crop growing season, the amount of land irrigated since more land can be irrigated during above‐average rainfall years with a limited amount of irrigation water than during below‐average years. The design used a limited water supply to irrigate a larger area than could be conventionally irrigated. A graded furrow field, 600 m long on a 0.3 to 0.4% slope was divided into three water management sections. The upper half of the field was managed as “fully irrigated.” The next one‐fourth was managed as a “tailwater runoff” section that utilized furrow runoff from the fully irrigated section. The lower one‐fourth was managed as a “dryland” section capable of utilizing runoff from either rainfall or irrigation on the wetter sections. The system was field tested for 3 years with grain sorghum (Sorghum bicolor(L.) Moench) on a Pullman clay loam (fine, mixed, thermic family of Torretic Paleustolls). Seasonal rainfall values were 224 mm in 1979, 103 mm in 1980, and 424 mm in 1981 as compared to the average of about 250 mm. The LID system was successful in increasing the utilization efficiency of irrigation water. Irrigation water added to a field can be transpired, evaporated, percolated, lost as runoff, or left stored in the soil after harvest. The LID system tended to reduce all losses other than transpiration. When 125 mm or 185 mm of irrigation water were added with the LID system, the evapotranspiration by grain sorghum was increased by an almost equal amount. This increased grain yield an average of 154 kg/ha for each 10 mm added irrigation, compared to 92 kg/ha for each 10 mm under conventional irrigation practices.