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Feed Intake Specifications in Defining Breeding Objectives
Abstract
Development of selection objectives for beef cattle requires consideration of the available feed resources. In production systems with access to liberal quantities of harvested feeds of relatively high quality, selection to improve economic efficiency will normally favor animals with high genetic potentials for the primary production traits. However, these highly productive types may also have relatively high maintenance requirements and be less efficient when nutrient restrictions prevent expression of genetic differences in appetite. This, important genotype × environment interactions may be observed. In range environments, animals may be restricted in the amount of forage that can be harvested. These restrictions usually arise from an inability of the animal to increase grazing times or geed intake rates beyond certain levels imposed by the sward and by innate animal requirements for rest and rumination. In addition, maintenance requirements are often elevated in range environments, thereby further limiting the increases in productive capacity that can be accommodated. Under conditions of nutrient restriction, antagonistic phenotypic relationships may also be observed among production traits (such as milk production and reproduction). The nature of these relationships must be considered in the development of selection criteria.
... Apparent differences between yearling and older ewes in the association between lamb BW and ewe NLB (Table 5) also suggested a possible structural association between these variables in yearling ewes. Models for structurally related traits were described by Wright (1960), discussed in the context of antagonisms between productive and reproductive traits by Notter (1986), rigorously modelled by Gianola and Sorenson (2004), and applied to productive and fitness traits in dairy cattle by de los Campos et al. (2006). Understanding inter-relationships among production, reproduction, and fitness traits is particularly important for development of breeding objectives. ...
This study estimated genetic parameters for ewe reproductive traits [number of lambs born (NLB) and weaned (NLW) per ewe lambing] and fecal egg counts (FEC) during the peri-parturient rise (PPR) for use in genetic evaluation of Katahdin sheep. Data included NLB and NLW for 23,060 lambings by 9,295 Katahdin ewes, 1,230 PPR at lambing (PPR0) for 750 ewes, 1,070 PPR at approximately 30 d postpartum (PPR30) for 611 ewes, BW at birth, weaning, and (or) post-weaning for 12,869 lambs, and FEC at weaning and (or) post-weaning for 4,676 lambs. Direct additive, permanent environmental, and residual (co)variances were estimated in univariate and bivariate animal models. Fixed effects included effects of ewe management group and ewe age for all traits, and, for PPR, a continuous effect of days between lambing and measurement. Effects of litter size on PPR0 and number of lambs suckled on PPR30 were included in univariate models but excluded from bivariate models for PPR and NLB or NLW. Heritability estimates in univariate models for NLB, NLW, PPR0 and PPR30 were 0.09 ± 0.01, 0.06 ± 0.01, 0.35 ± 0.06, and 0.24 ± 0.07, respectively. Estimates of permanent environmental variance as a proportion of total phenotypic variance were 0.02 ± 0.01 for NLB, 0.03 ± 0.01 for NLW, 0.05 ± 0.06 for PPR0, and 0.13 ± 0.07 for PPR30. Direct additive, phenotypic, permanent environmental and residual correlations between NLB and NLW were 0.88 ± 0.03, 0.74 ± 0.004, 0.54 ± 0.15, 0.74 ± 0.003, respectively; corresponding correlations between PPR0 and PPR30 were 0.96 ± 0.07, 0.46 ± 0.03, 0.98 ± 0.50, 0.18 ± 0.05, respectively. The additive genetic correlation (rd) between ewe reproductive traits and PPR ranged from 0.12 to 0.18. Estimates of rd between lamb body weight (BW) and subsequent ewe NLB and NLW ranged from 0.07 to 0.20, and those between PPR and lamb BW ranged from -0.03 to 0.29. The rd between ewe reproductive traits and lamb FEC ranged from 0.27 to 0.40, and those between PPR and lamb FEC ranged from 0.56 to 0.77. Correlations between maternal additive effects on BW and direct additive effects on PPR were low (-0.08 to 0.10), and those between maternal additive effects on BW and direct additive effects on ewe reproductive traits were variable (-0.36 to 0.11). We conclude that FEC in growing lambs and peri-parturient ewes are controlled by similar genes and that modest, but manageable, genetic antagonisms may exist between FEC and ewe productivity.
Thesis (Ph.D. (Animal, Wildlife and Grassland Sciences))--University of the Free State, 2004. Bibliography: leaves 166-188.
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