Environmental and genotype effects on fertility in a commercial beef herd in central Queensland

Abstract

Joining liveweight, proportion pregnant and calving date were recorded from relatively low fertility (RLF) and relatively high fertility (RHF) tropically adapted genotypes. RLF genotypes were F2 et seq. Brahman x British crossbreds and Brahman back crosses while RHF genotypes were F1 Brahman x British and Africander infused crossbreds. Year effects had the largest and most consistent influence on joining liveweight, proportion of cows pregnant and calving date. RLF genotypes had a lower proportion pregnant than RHF genotypes in yearling heifers, 2-5-year-old and >9- year-old lactating cows, but there was no difference in the 6-8-year age group. Calving dates tended to be later in the RLF genotypes in the 2-5-year age groups, but were the same as RHF genotypes in older age groups. Annual variation in joining liveweight was explained by May-October rainfall, which was an indication of pasture productivity levels. The proportion of cows pregnant was related to joining liveweight; to achieve a high proportion in the RHF genotypes, target joining liveweights of about 275 and 375 kg were indicated for yearling heifers and lactating cows, respectively. There was no difference between RLF and RHF in the 6-8-year lactating age group, but in other age groups RLF genotypes tended to require an extra 25-75 kg joining liveweight to approximate the proportion pregnant in RHF genotypes over the lower and mid-liveweight ranges. However, at highest joining liveweights, RLF had a lower proportion pregnant than did RHF genotypes. Calving date influenced the proportion that became pregnant during the subsequent joining season, young cows being more sensitive to calving date than old cows, and RLF more sensitive than RHF genotypes.