Three and a half thousand lambs from Border Leicester x Merino ewes mated to 133 sires from five Poll Dorset, one White Suffolk, one Siromt, two Meridale and four Merino studs were slaughtered, their carcasses halved and one side divided into six primals. Subcutaneous fat was dissected from all six primals, and bone from only the three rear primals. There were four slaughter groups: average slaughter weights of 30 and 35 kg for ewes and 35 and 45 kg for cryptorchids. Heritabilities and phenotypic and genetic correlations for all traits measured (>50) are published in an appendix. Where comparisons were available, estimates were similar to those for purebred animals. Genetic parameters for various assessments of fat were similar except for channel and omental fat. The GR fat depth was the best predictor for total subcutaneous fat, cannon bone length for total bone, and eye muscle area for total soft tissue. Carcass weight and GR appear to be the most important measurements for use in selection for breeding of sires for the prime lamb industry. Slaughter weight and fat depth at the C site could be used as suitable alternatives on live animals. Production of lean meat is not likely to be increased greatly by including measurements other than liveweight and GR in a selection index. Of the other measurements bone length and eye muscle measurements showed most promise.
Sixty-five Poll Dorset x (Border Leicester/Merino) ewe lambs. were grown at pasture on one of four growth paths commencing at mean age of 24 weeks and a mean liveweight of 37-5 kg. These were, continuous growth between 24 and 34 weeks (HH), maximum possible growth for 6 weeks followed by weight loss for 4 weeks (HL), no gain for 6 weeks followed by rapid gain for 4 weeks (LH) and weight maintenance (LL). Lambs were slaughtered as soon as possible after LH and HL lambs were the same full weight. There were significant differences between treatments in average growth rate, liveweight, hot carcass weight (HCW) and dressing percentage. After adjustment for HCW, LH had significantly (P < 0.05) less omental and kidney fat than HH, and there was a strong tendency for less subcutaneous fat, intermuscular fat and a lower tissue depth over the twelfth rib (at the GR site). Eye muscle area was significantly larger in LH than HH, but this did not translate to a significant difference in dissectible 'soft tissue' measurement. The Warner Bratzler shear value for LL in the M. longissimus dorsi was significantly higher than LH and HH. There were no differences between treatments for pH, Warner Bratzler shear values of the M. semimembranosus, cooking loss, or the colour measurements using a Minolta chromameter of L* (lightness) or b* (yellowness). The difference in a* (redness) between HL and LH was significant. Warner Bratzler shear values for M. longissimus dorsi and M.semimembranosus were significantly higher at 4�C than 10�C. The a* was significantly higher at 10�C, but there were no differences between L* and b*. There were no interactions between the post-slaughter chilling regime and the pre-slaughter nutritional management. It was concluded that the growth path had little effect on meat quality unless the lamb had been continuously on a restricted diet. Therefore there was some potential for the use of growth paths to manipulate carcass composition, and further research into management systems at pasture is recommended.
Lambs from the Poll Dorset x (Border Leicester x Merino) cross were raised from a late lambing in 1983 (a year when pastures grew unchecked and had high clover content) and 1984 (a year with intermittent dry periods and a low clover content in pastures). Multivariate repeated measures analyses were used to compare growth rates for rams, wethers and ewes within 2 periods of growth in 1983, and 4 in 1984. In all stages rams grew significantly faster than wethers and ewes, whereas wether growth rate exceeded that of ewes in only 3 stages. In general, when pasture availability and quality was high, rams grew faster than wethers by 0.3 kg/week and ewes by 0.5 kg/week. The rams grew 0.16 kg/week faster when pasture availability was limiting for a period in winter but lost significant (P<0.01) body condition, whilst ewes maintained both weight and condition. When good quality feed was again available, rams gained more weight than wethers, as did wethers compared with ewes, while rams and wethers gained more condition than ewes. Carcass weight and fatness, measured either as indicators of fat (GR measurement or fat score) or as total subcutaneous fat, could be predicted from assessments made on live lambs. Full liveweight, sex and year explained 41% of variation in GR, 48% in fat score and 79% in subcutaneous fat. Condition score, sex and year explained 45% of variation in GR, 52% in fat score and 57% in subcutaneous fat. Full liveweight, condition score, sex and year explained 80% of the variation in subcutaneous fat, 47% in GR and 57% in fat score. An interaction of sex with full liveweight in predicting carcass weight and subcutaneous fat indicated that as carcass size increased, the sex of the lamb became an increasingly important consideration when assessing characteristics important in describing lamb carcasses. Dressing percentage decreased by 0.08/kg as liveweight increased. This result was consistent over the 2 years, with opportunities for error being assessed and eliminated in the second year. Notwithstanding that this was an unimportant loss in real terms, published information showed an increase in dressing percentage with liveweight.
These experiments evaluated in northeastern Victoria the factors affecting the severity of browsing damage to eucalypts by sheep. We aimed to develop management options for integrating sheep grazing in agroforestry systems. One-year-old tree seedlings were planted into annual pasture and grazed at high stocking rates (44 sheep/ha) for up to 1 week when the pasture was abundant. In experiments 1 and 2, crossbred weaners almost totally defoliated Eucalyptus camaldulensis (river red gum) seedlings after 96 h grazing; damage from older Merino wethers was minimal. Of the 4 eucalypt species tested for relative acceptability or palatability to crossbred weaners, E. camaldulensis was the most heavily browsed and E. globulus spp. bicostata (southern blue gum), least. The height of the pasture immediately surrounding the seedlings, and therefore visibility of the seedling to the sheep, did not affect browsing extent. In experiment 3, potential repellents were applied either on the ground around E. camaldulensis seedlings or sprinkled on the foliage. Six repellants provided short-term protection from crossbred weaner sheep; Replex 1 and 3, and egg and paint were the most effective. After 4 days grazing, seedlings treated with Replex 3 had sustained 24% mean browsing damage compared with 90% for the control.
A technique is described for assessing the profitability of selecting sheep for wool production. A model of the breeding system incorporating three submodels is developed: one to represent the flock from which the selection intensities for rams andewes are calculated; a second to determine gains made in wool production and the year in which they accrue; and the third uses discounting procedures to examine the economics of selecting for wool production. Flock and genetic parameters were set at 'average' values for an Australian Merino wool producer and gains were determined for a range of lambing percentages. Three measures were used to evaluate profitability; the net present value of the programme which shows overall profits, the internal rate of return, which gives the earning rate of the programme, and the payback period, which identifies the number of years before early costs are repaid. A sensitivity analysis was used to examine changes in the three measures of profitability resulting from changes in interest rate, wool price, costs for labour and computer analysis, wool testing costs and lambing percentage. Profitability was sensitive to changes in all parameters except wool testing charges, and of the remainder the producer has a degree of control only over labour costs. A programme can be profitable, however, if wool price and lambing percentage are adequate and costs can be kept at reasonably low levels. Simple breeding programmes should be studied in which labour inputs are kept to a minimum or where costs are confined to studs (nuclei) and genetic gain disseminated by the sale (migration) of rams.
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