Genetic and phenotypic aspects of live weight, body condition score and reproductive success of beef cows in New Zealand : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Animal Science at Massey University, Manawatū, New Zealand

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Massey University
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The productivity of spring-calving beef cows under New Zealand hill country conditions is dictated by the seasonality of the system. With the calf being the primary production output, reproductive success of the cow, alongside growth and carcass merit of the calves, is essential. The objective of this thesis was to quantify live weight (LWT), body condition score (BCS) and reproductive performance of beef breeding cows in New Zealand’s pastoral farming systems, both on a genetic and phenotypic level. This information was used to compare the profitability of cows that differ in their mature live weight (MWT) and other correlated traits exposed to variable feed availability. The BCS of cows is a relevant indicator trait for productivity of the maternal herd on a phenotypic level. Cows in good body condition, particularly in the period leading up to mating, are more likely (P < 0.001) to get pregnant. The curvilinear relationship indicates that most improvement may be obtained by reducing the percentage of low conditioned cows in the herd (75.7% pregnancy rate at BCS 4.5 versus 93.3% pregnancy rate at BCS 8). From a genetic perspective, the correlation between BCS and pregnancy rate was low (rg = -0.10). Pregnancy rate was considered as a separate trait for each of 15-month-old heifers, first-lactation 2-year-old cows and mature cows. Genetic variation and heritability were low for 15-month-old heifers and mature cows (h2 ≤ 0.06), but greater for 2-year-old cows (h2 = 0.12-0.14), indicating that 2-year-old cows offered the greatest potential for selection on pregnancy rate. At all ages, heritabilities were greater for BCS (h2 = 0.26) and for MWT (h2 = 0.48) than for pregnancy rate. Placing downward pressure on MWT may decrease the ability of cows to reach BCS targets (rg = 0.24) which can impact on subsequent reproductive success. Most growth and weight traits in finishing cattle had only limited genetic correlations with rebreeding ability of female herd replacements (rg = 0.11–0.32) or cow BCS (rg = 0.12–0.36) but high genetic correlations were observed with MWT (rg = 0.45–0.92) in the current study. Fat and muscle traits recorded on finishing animals were negatively genetically correlated with MWT (rg = -0.40 - -0.19) but were only weakly correlated with cow BCS (rg = 0.02–0.25). Comparison of simulated cows selected on the basis of MWT estimated breeding values (EBV) from within the current New Zealand Angus population indicated that there was little variation in profitability among different cow MWT types under conditions where supplementary feed in terms of increased pasture allowance was introduced to prevent excessive loss of condition. The cow MWT types in the simulation model differed in MWT EBV and correlated traits such as calf size, and adjustments in stocking rate were made to reflect changes in feed requirements per cow. Overall, this thesis provided evidence that for cow-calf production systems where the primary focus is on enhancing the maternal performance of the cow, selection emphasis should be on improving rebreeding ability of 2-year-old cows as the most heritable reproduction trait, and on BCS of cows. Within the limitations of this study, cow types that differed in their MWT had only minor differences in profitability when simulated under a variable feed supply, such that selection strategies may lean towards prioritising the performance of finishing animals while taking advantage of those larger calves from bigger cows and accepting a correlated increase in MWT. This information can be used to develop guidelines for future genetic evaluation programs for beef cattle in New Zealand.
Beef cattle, Breeding, Physiology, New Zealand, Genetics