Indirect selection for resistance to clover infertility in Merino ewes

Abstract

Animal Production in Australia 1998 Vol. 22 INDIRECT SELECTION FOR RESISTANCE TO CLOVER INFERTILITY IN MERINO EWES K.P. CROKERA, N.R. ADAMSB, M.A. JOHNSA and M.R. SANDERS A B C BC Animal Research and Development Services, Agriculture WA, Bentley, WA 6983 CSIRO Division of Animal Production, Private Bag, Wembley, WA 6014 Present address: 5 Meharry Road, Hillarys, WA 6025 It has been shown that resistance to the development of clover infertility in Merino ewes is heritable (realised heritability of the ewes fertile lifetime = 0.73). This study examined whether indirect selection for resistance, based on the lambing performances of ewes, occurred in flocks. Ewes and rams were collected from two farms in 1988 where consistently good reproductive performances had been recorded on high- (Jingalup, H) or low- (Narambeen, L) oestrogenic pastures for a number of years. The sheep were taken to Badgingarra where the 200 ewes and 8 rams from each farm were subsequently joined as separate groups. Ewes from each farm then lambed on either a green high-oestrogenic pasture (Dinninup sub clover, 0.87% formononetin, 0.55% genistein, 0.67% biochanin A) or on a green non-oestrogenic pasture (a forage oat crop). The progeny were grazed on these pastures during each subsequent growing season throughout the study which was finished in February 1994. Rams were joined with the ewes each December. As shown in Table 1, grazing oestrogenic pasture depressed, although not significantly, the lambing performances of the ewes after the first year, but not the pregnancy rate, which usually is lower in ewes grazed on oestrogenic sub clover pastures. The shapes of the vulvas in late spring, and histology of the cervixes after slaughter of the ewes following the last joining, were also measured but these measurements did not show significantly less change for the ewes bred from those collected from the farm with the high-oestrogenic pastures compared with the ewes bred from those collected from the farm with the low-oestrogenic pastures. Table 1. The reproductive performances of ewes bred from ewes grazed on high- (H) and low- (L) oestrogenic pastures during their lifetimes Ewes Origin a 1991 Pasture grazed b 1992 d 1993 LB (%) EP (%) LB (%) 1994 EP (%) L P (%) e EP (%) L B (%) c EP (%) H H L L Chi square a b c d e f Oestr. Non-oestr. Oestr. Non-oestr 81.8 90.0 72.0 75.0 nsf 100.0 103.3 76.0 79.2 ns 89.5 96.0 93.3 91.7 ns 110.5 136.0 110.0 137.5 ns 92.9 95.0 95.7 90.0 ns 93.3 115.0 95.7 115.0 ns 84.6 78.9 95.5 88.9 ns 92.3 121.1 127.3 144.4 ns Origin - original dams collected from farms with high- (H) and low- (L) oestrogenic pastures Pasture grazed - the ewe progeny grazed on either Dinninup sub clover or fodder oat pastures EP - percentage of ewes pregnant LB - percentage of lambs born to ewes present at lambing LP - percentage of lambs in utero when ewes slaughtered Chi square not significant P>0.05 It is concluded that indirect selection based on the previous lambing performances of ewes has not been strong enough to result in resistance to the effects of isoflavones early in life. We thank R. Young and H. Williams who provided the initial ewes and rams for this study. The technical support of the Manager and staff at Badgingarra Research Station was crucial for the conduct of the work. 409