HERITABILITIES AND GENETIC CORRELATIONS OF BODY-WEIGHT, TESTIS GROWTH AND EWE LAMB REPRODUCTIVE TRAITS IN CROSSBRED SHEEP

Heritabilities (h(2)) and genetic correlations (r(G)) involving body weights, measures of testes size and ewe lamb reproduction were calculated using 953 spring-born lambs produced during formation of a three-breed composite population containing 0.50 Dorset, 0.25 Rambouillet and 0.25 Finnish Landrace breeding. The h(2) of body weight ranged from 0.2 to 0.3 between 45 and 150 days of age, but increased to 0.41 at the start of breeding of ewe lambs at about 6 months of age. Additive maternal effects were not important, perhaps because intensive housing encouraged cross-stickling. Testes size was measured as scrotal circumference (SC), SC with linear adjustment for weight (SCW) and SC divided by the one-third power of body weight (RSC). The h(2) of SC traits was highest at 90 days (0.52 for SC, 0.62 for RSC and 0.57 for SCW), and h(2) for age at maximum SC growth rate (i.e. TI, the inflexion point of a logistic SC growth curve) was 0.49. The h(2) of ewe lamb fertility in autumn was 0.09 but the h(2) for frequency of remating the following spring was 0.41. The h(2) for ewe lamb litter size in these data tons zero. At 90 days, r(G) between alternative measures of testes size exceeded 0.86; r(G) with body weight were 0.67 for SC, 0.53 for RSC and -0.74 for TI. The r(G) with ewe lamb fertility for 90-day weight, SC, RSC and SCW and for TI were -0.25, 0.20, 0.25, 0.38 and -0.32, respectively. The r(G) for these traits with spring remating were 0.34, 0.34, 0.26, 0.10 and -0.48, respectively.