Agriculture and Food Systems - Theses
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ItemPhysiological and morphological responses of several genotypes of tropical and sub-tropical maize (Zea mays L.) to water and nitrogen deficit during the early vegetative phase( 1997)Intermittent drought combined with nitrogen stress during early growth of maize crops in the tropics limits maize production in developing countries. Plant morphological and physiological responses of different maize genotypes to two aspects of stress, water deficit and nitrogen stress, were examined during the growth of several maize genotypes. Eight maize genotypes including three inbred lines (Gn 160, Gn 106, Gn 175), three single crosses (Gn 160 x Gn 175, Gn 106 x Gn 175, Gn 106 x Gn 160) and two open pollinated cultivars (Tuxpeno Sequla Cycle 0, drought sensitive; Cycle 8, drought resistant) were examined for drought tolerance during early growth. Variables used for assessing tolerance were dry weight of shoot and root, leaf area, relative water content, water potential and stomatal conductance which were measured in both well-watered and the respective droughted treatments for each genotype. Under growth room conditions, the level of severe drought imposed caused a substantial reduction in growth of eight maize genotypes, relative to the respective well-watered conditions. The single cross Gn106 x Gn160 revealed drought tolerance traits in shoot and total biomass, which were higher than the other single crosses. It is concluded that this single cross is more drought-tolerant than the other single crosses during early growth stages. Tuxpeno Sequla Cycle 0 and Cycle 8 had similar shoot and total biomass under both water regimes, and did not show differences in drought tolerance during early growth. There was no clear indication of transmissability of drought tolerance from parent lines to their respective single crosses. The combined effects of drought and timing of N stress during early growth on morphological and physiological traits, which were altered during later growth and yield components, were investigated in two related tropical maize cultivars, Tuxpeno Sequla Cycle 0 (Tux. CO) and Cycle 8 (Tux. C8), which were selected for drought sensitivity (Tux. CO) and tolerance (Tux. C8), during flowering under glasshouse conditions. Total biomass at maturity and green leaf number below the ear at grain filling were similar between Tux. CO and C. 8 under different water regimes. Increased grain yield and harvest index in Tux. C8, compared with Tux. CO, resulted mainly from increased kernel number per row and rows per ear. Tux. C8 was not only higher in yield but also higher in N use efficiency than Tux. CO when experiencing moisture and N stress during early growth. Shoot biomass under moderate drought versus well-watered conditions was found to be more sensitive than root biomass in both Tux. CO and C8. Under drought compared with well-watered conditions, stem biomass was more sensitive than leaf biomass. Leaf area in the high-N treatments was more sensitive to water deficit than in low N treatments, as indicated by the fact that droughted plants in the early N application (NE) and split (NS) treatment produced lower leaf areas than well-watered plants, while in the control (NO) and late N (NL) treatments there was no significant difference in leaf area between drought and well-watered conditions. Early N application resulted in a shorter time from sowing to anthesis than later N application. Clearly N stress is the main limiting factor for growth and yield of maize under moderate level of water deficit. This study revealed that maize plants require N for growth and dry matter accumulation as early as immediately after seedling emergence even under low N input conditions (75 kg N per hectare). Increased grain N content and nitrogen harvest index for a split N application resulted from increased N content in the stem and leaf Even under low N input conditions i.e. the application equivalent to 75 kg N/ha in this experiment, N still needed to be supplied as a split application, in order to increase N use efficiency. Nitrogen harvest index under well-watered conditions was higher than that under drought, indicating that water deficit also had an effect on nitrogen harvest index of both cultivars (CO and C8) under different level of N treatments.
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ItemProduction of heavy weight cryptorchid and wether lambs( 1996)Declining Iamb consumption remains a major problem facing the Australian prime Iamb industry. For Iamb to retain its market share and to remain a viable commodity in the future, leaner Iambs must be produced and marketed. Recent industry initiatives therefore focussed upon the production and marketing of Iambs to meet carcass specifications of 18-26 kg with a fat depth of 6 to 15 mm at the GR site. Emphasis was placed upon the production of lean carcasses weighing more than 22 kg, which are otherwise referred to as Elite Iambs. The development of production, management and marketing systems to produce and market Iamb carcasses of Elite Iamb specifications, both domestically and overseas, was the focus of this research program. The major objective of this experiment was to determine the potential of producing high and consistent quality meat from cryptorchid and wether Iambs meeting carcass specifications for Elite Iambs when slaughtered from six to eighteen months of age. One hundred and sixty three second cross Poll Dorset x Border Leicester/Merino cryptorchid and wether Iambs, born in May 1991 at Rutherglen Research Institute, Rutherglen, were grown and managed on annual pasture at an average liveweight of 50 kg prior to slaughter. Lambs were allocated to one of seven slaughter groups and finished every two months from 6 to 18 months of age and the influence of sex, age and nutrition on carcass composition, meat quality attributes and fat quality and consistency of cryptorchid and wether Iambs examined. Cryptorchid and wether Iambs slaughtered from 8 to 18 months of age produced carcasses meeting targeted specifications of greater than 22 kg with 6-15 mm of fat at the GR site. Lambs slaughtered at six months of age did not achieve the target liveweight of 50 kg at slaughter due to poor seasonal conditions which influenced the quality and availability of annual pasture in spring 1991. Cryptorchid Iambs had a lower dressing percentage and produced carcasses with a lower GR measurement than wethers at a cold carcass weight (CCW) of 23.5 kg. Cryptorchid Iambs exhibited growth rate advantages over wethers only when nutrition and seasonal conditions did not limit Iamb growth. When compared at the same fasted liveweight, cryptorchid Iambs deposited less internal fat than wethers. Cryptorchid carcasses also had a lower proportion of subcutaneous fat present on all primals, higher proportions of soft tissue and bone in the hindquarter and the EMA of the M. longissimus thoracis was significantly larger compared with wethers at a CCW of 23.5 kg. The proportion of CCW in the leg, chump and ribloin was not significantly influenced by Iamb sex. Nutritional management of lambs significantly influenced the proportion of subcutaneous fat on all primals. Meat quality was significantly influenced by Iamb sex however results obtained did not suggest that meat from cryptorchid Iamb carcasses was of inferior quality compared with wethers as only small differences in tenderness, intramuscular soluble collagen content, meat flavour, meat colour, intramuscular fat content and cooking loss were found. In addition, age and nutrition significantly influenced meat quality, however, consistent relationships were not found. The incidence of yellow soft subcutaneous fat was higher in cryptorchid Iamb carcasses than those from wethers with cryptorchids also producing subcutaneous fat with significantly higher levels of total unsaturated fatty acids and lower levels of stearic, palmitic and total saturated fatty acids compared with wethers. Age and nutrition also influenced the fatty acid composition of subcutaneous fat with Iambs fed lupins having significantly higher linoleic acid levels in subcutaneous fat. However, the magnitude of these differences did not explain differences found in fat quality and consistency between cryptorchid and wether carcasses slaughtered at different ages. Significant, though small, differences were found due to sex in both the triglyceride and phospholipid components of intramuscular fat. As higher levels of polyunsaturated fatty acids were found in the phospholipid component of intramuscular fat compared with cryptorchids, this may affect the keeping quality of meat from wether carcasses. Finally, although significant differences due to age were found in the fatty acid composition of intramuscular fat, these differences were not consistent with age and more likely reflected differences in nutritional management between slaughter groups. This study demonstrated that cryptorchid and wether Iambs turned off at heavy market weights on an improved plane of nutrition over a 12 month period produced carcasses meeting Elite Iamb specifications and yielded high quality, tender meat.