Frequency of the prolificacy gene in flocks of Indonesian thin tail sheep: a review.
Indonesian thin tail (ITT) sheep have a major prolificacy gene (FecJ(F)), the frequency of which is higher in the new born lambs, than in the remainder of the flock when mating is random, because carrier ewes produce more progeny than do non carriers. The frequency of the gene may vary between flocks, but remains relatively stable in flocks with established husbandry procedures. The countermanding selection pressures maintaining the equilibrium value for the frequency of FecJ(F) are mainly those deriving from higher mortality rates of lambs in larger litters. Embryo survival is not significantly different across the range of ovulation rates in ITT ewes, in contrast to observations in other prolific breeds. Generation intervals and the incidences of metabolic and infectious diseases in ewes carrying larger litters can also affect the frequency of FecJ(F) in flocks. In turn, each of the factors affecting the frequency of FecJ(F) is modulated by the level of nutrition and management in each flock. The distribution of prolificacy genotypes in the ewes of a standard flock is calculated as FecJ(F)FecJ(F)/12; FecJ(F)FecJ(+)/44; FecJ(+)FecJ(+)/44 giving a frequency of 0.34 for FecJ(F). The frequency of FecJ(F) is then 0.43 in the lambs at birth, when their numbers have been amplified in the carrier ewes. There are heavier metabolic demands on ewes carrying larger litters and the foetuses constitute a higher proportion of weight gain during pregnancy. Consequently, more of the lambs of carrier ewes are smaller and weaker at birth, and the reserves of the ewes for colostrum and milk production are depleted. When low lamb survival rates in larger litters are considered, the frequency of FecJ(F) falls to 0.35 in the lambs at weaning in lean years and 0.38 in middling years. At a high level of husbandry, ewe weight gains during pregnancy and lamb survival rates improve substantially, and after 3 years, the frequency of FecJ(F) in the lambs at birth is estimated to have risen to 0.49; and 0.47 at weaning.
Roberts
JA
Schultz Road, Witta, via Maleny 4552, Queensland, Australia