|Van vleck, Lloyd|
Submitted to: Small Ruminant Research
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/31/2001
Publication Date: 1/17/2002
Citation: Rosati, A., Mousa, E., Van Vleck, L.D., Young, L.D. 2002. Genetic parameters of reproductive traits in sheep. Small Ruminant Research. 43(1)65-74. Interpretive Summary: Reproductive rate in sheep is a major component of profitability. Records of 7,642 from five pure and two composite breeds maintained at USMARC were used to estimate genetic parameters necessary to develop selection indexes to be used to improve reproductive rate. A total of 13 traits were studied including conception, birth, and weaning rates as well as litter weight at weaning. Estimates of heritability were low and suggest that improvement of reproduction by selection would be slow. The study also found important genetic correlation among the traits. The estimates of heritabilities and genetic correlations can be used to determine an index for optimal select- ion for economic returns and to predict direct and correlated responses to selection for any trait or combination of these reproductive traits.
Technical Abstract: Reproductive traits from 7,842 ewes ewre recorded from 1975 to 1983. The ewes were of five breeds [Dorset (D), Finnsheep (F), Rambouillet (R), Suffolk (S) and Targhee (T)] and two composite lines [C1 (1/2 F + 1/4 R + 1/4 D) and C2 (1/2 F + 1/4 S + 1/4 T)]. Genetic parameters were estimated for six basic and seven composite traits. The basic traits were conception nrate (CR), total number of lamb born (NLB), number of lambs born alive (NLBA), number of lambs alive at weaning (NLAW), litter mean weight per lamb born (LMWLB) and litter mean weight per lamb weaned (LMWLW). The composite traits were ratio of lambs surviving to weaning relative to NLB (LSW = NLAW/NLB), number of lambs born per ewe exposed (NLBEE = CR*NLB), number of lambs weaned per ewe exposed (NLWEE = CR*NLAW), total litter weig ht at birth (TLWB = NLB*LMWLB), total litter weight at weaning (TLWW = NLAW*LMWLW), total litter weight at birth per ewe exposed (TLWBEE = CR*NLB* *LMWLB) and total litter weight at weaning per ewe exposed (TLWWEE = CR* NLAW*LMWLW). Year, age of ewe, breed of ewe, hormone treatment and season of breeding were used as fixed effects. Direct and maternal genetic effects, permanent environmental effects of ewe and mate of ewe were con- sidered to be random effects. Estimates of variances due to animal genetic and permanent environmental and maternal genetic effects were mainly small due to high influence of environmental factors on reproductive traits and to non-normal distributions of traits. Mate of ewe effects were not important. Important genetic correlations were found between some traits. The estimates of genetic correlations among traits may provide a basis for deriving selection indexes for reproductive traits.