POPULATION ANALYSIS OF A PUREBRED HEREFORD AND A MULTIBREED SYNTHETICBEEF-CATTLE HERD

Lifetime records of females born from 1966 to 1975 were used to estima te and compare population parameters of a purebred Hereford (HE) and a multibreed synthetic (SY1) beef cattle herd raised under a stringent culling policy whereby heifers and cows failing to wean a calf each ye ar were culled. Population size averaged 118 cows, 39 heifers and 155 cows, 56 heifers a year for HE and SY1, respectively. The SY1 was a mu ltibreed composite breed group with an average breed composition of 33 % Charolais, 33% Angus, and 20% Galloway, and the remainder from other beef breeds. The 2 herds were raised under the same management. Nine life table statistics were studied: age-specific survivorship, age-spe cific survival rate, mortality rates (Qx), expected herd life, age-spe cific birth rate, reproductive value, net reproductive rate (Ro); inst antaneous rate of population increase (r) and generation interval (T). Differences were obtained between the herds for the age-specific life table statistics, with SY1 having higher values (except for lower Qa values) than HE. SY1 had higher means than HE for Ro (1.57 +/- 0.11 ve rsus 1.21 +/- 0.15; p < 0.07) and r (0.09 +/- 0.01 versus 0.03 +/- 0.0 2; p < 0.04), indicating a faster rate of population growth in SY1. Th e value of T for SY1 was higher (p < 0.01) than that for HE (5.09 +/- 0.11 versus 4.25 +/- 0.19 years). The results indicate that the same m anagement and culling policy may result in different life. table stati stics, which in this study was possibly due to the influence of hetero sis for calf survival in the multibreed composite SY1 herd. Over time the stringent culling policy had the effect of reducing Ro, r and T va lues to the point where herd size in the HE herd could not be maintain ed (Ro < 1 in the 1972 and later cohorts).