ABSTRACTA study was conducted to determine genetic and phenotypic parameters of reproductive traits from 2773 lambings of a semi-intensively managed breeding flock of Yankasa sheep at the National Animal Production Research Institute (NAPRI), Shika, situated within the subhumid zone of Nigeria. Edited records of 2204, 2490, 1225, 1461, 1196, 1204 and 247 lambings were used to analyse for environmental effects on litter size (LS), birth weight (BWT), litter birth weight (LBWT), 90-day weaning weight (WWT), gestation length (GL), lambing interval (LI) and age at first lambing (AFL), respectively. Least-squares means (±SE) were 1.22±0.01 (LS), 2.48+0.01 kg (BWT), 3.57±0.03 kg (LBWT), 10.58±0.11 kg (90-day WWT), 151.37+0.01 days (GL), 249.81+2.72 days (LI), and 575.00±9.55 days (AFL). Litter size was significantly (P<0.001) affected by parity, season and year of birth. Multiparous ewes and ewes lambing in the dry season were superior in litter size to primiparous ewes and ewes lambing in the rainy season. Ram lambs were consistently heavier than ewe lambs (P<0.001) by 0.12 and 0.78 kg respectively at birth and at weaning. A similar trend was observed with single and twin bom lambs, the difference (P<0.001) being 0.39 and 2.32 kg in favour of single born lambs at birth and at weaning respectively. Lambs from multiparous ewes had higher body weights than lambs from primiparous ewes, likewise rainy season born lambs than dry season bom lambs. Gestation length was significantly affected by parity (P<0.05) and year of birth (P<0.001), but not by litter size, sex and season of birth (P>0.05). Younger ewes had shorter GL than older ewes, being 150.94 vs 151.70 days for primiparous and sixth parity ewes. Parity, litter size and sex of lamb insignificantly (P>0.05) affected LI, while the effects of season (P<0.01) and year of birth (P<0.001) were highly significant. Ewes lambing in the rainy season had shorter LI than ewes lambing in the dry season (249.64 vs 249.99 days). AFL was significantly affected by type of birth (P<0.05), season and year of birth of ewe (P<0.01). AFL was attained earlier in single than twin bom ewes. Additive environmental adjustment factors for effects of parity of dam, type of birth, sex of lamb and season of birth for LS, BWT, and 90-day WWT were derived. Heritability estimates for LS from daughter-dam regression was 0.07. Using paternal half-sib analysis. BWT, LBWT, 90-day WWT, GL as dam trait. GL as foetal trait, LI and AFL were estimated at 0.41±0.08, 0.12±0.07, 0.59±0.12, 0.15±0.12, 0.21+0.08, 0.73+0.26 and 0.99±0.38 respectively. With the exception of low heritability estimate for litter size, most values were moderate to high suggesting moderate to high genetic responses to selection for these traits. Estimates for LI and AFL were, however, considered inflated and attributed to dominance and epistatic effects. There were high genetic and phenotypic correlations (0.69±0.16 vs 0.36) between BWT and 90-day WWT indicating a high positive correlated response to selection for these traits. Genetic correlations between LS and BWT, LS and 90-day WWT, LS and GL, and LBWT and GL were -0.08±0.35, -0.25+0.33, -0.11±0.32 and 0.07+0.27 respectively. Corresponding phenotypic correlations were -0.14, -0.34, 0.02 and 0.09. Generally, genetic and phenotypic correlations for most traits were moderate to low, and /or negative
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