Death and reproductive rates of Sheep in relation to flock culling and selection

Summary

The records of a number of commercial sheep flocks, kept in such manner as permitted the collection of reliable data relating to the births, deaths, and other causes of decrement and accretion of flock numbers, have been used to compile tables of mortality, ewe and lamb survival, selection and culling factors, and similar information derivable therefrom. The number of breeding ewes “exposed to risk” totalled 83,113 and observations extended over a period of 6 years, during which 77,493 lambs were born and survived to docking, yielding a mean lambing percentage of 93.9, expressed as the number of lambs docked and marked in relation to the number of ewes mated. Ewe deaths numbered 7,042, and lamb deaths prior to 1½ years of age totalled 8,056, or 10.4% of those docked.

Regression analysis indicated that an equation of the form: log. q_x = 0.156 x + 0.24, where “q_x” is the death-rate per cent at age (x), fitted the basic data with a correlation coefficient of 0.937.

A formula, embodying derived factors relating to the number of lambs docked per ewe mated, the probability of hogget survival to breeding age, and the subsequent mortality year by year of ewes in the breeding flock, was used to illustrate the quantitative relationships which are of importance in the annual casting of ewes for age and in determining the degree of selection of young breeding ewes necessary to maintain a proper balance in the breeding flock between the various decrementai and accretional effects.