Maternal Productivity in Beef Cattle: The impact on the female herd of genetic selection for a divergence in fatness or feed efficiency

Michael Laurence

Beef cows, particularly as part of seed stock and cow/calf enterprises, are farmed extensively in temperate environments in southern Australia. The fundamental premise of a productive beef enterprise is a calving interval of 365 days. However, production system efficiency can also be determined by comparing inputs, namely megajoules of energy in the form of food, to outputs, namely the number of kilograms of beef sold each year. This thesis examined two heritable and economically desirable traits and the impact that selection to improve these traits had on beef herd productivity – defined in this thesis as Maternal Productivity. Selection for reduced fatness, achieved by selecting animals on the basis of low Rib Fat EBVs, can increase profitability because of consumer demand for lean meat and because sellers of cattle for slaughter are penalised if the carcasses have too much fat on them. Selection for an improvement in feed-efficiency through the trait Net Feed Intake (NFI) can improve profitability because it allows an increase in stocking rates or a reduction in the number of megajoules supplied to the herd for the same level of production. There are concerns about the continued selection for these traits, particularly when energy supply is restricted such as in times of drought. There is evidence from other intensive production industries that single trait selection can compromise other desirable and necessary traits and that reduced body fatness reduces fertility in the long term. The impact of selection for fatness or feed-efficiency on Maternal Productivity was reported in this thesis. Two hundred Angus heifers, selected for a divergence in either fatness (Fat vs. Lean) or feed-efficiency (high-NFI vs. low-NFI), were subjected over two breeding cycles to either a high- or a low-nutritional treatment on an extensive grazing system in the south-west of Western Australia. Lean animals were phenotypically leaner than Fat animals at all stages of the breeding cycle and had higher predicted carcass yields than Fat animals, confirming the assumed economic benefit for selection for leanness. This result also confirmed that using Estimated Breeding Values to select for a divergence in fatness works. Low-NFI animals were phenotypically leaner than high-NFI animals at all stages of the breeding cycle and had higher predicted carcass yields than high-NFI animals. This result showed that differences in fatness between high- and low-NFI cattle previously identified in finished, grain-fed animals, persisted in the female grazing herd over two parities. There were no differences between experimental Genotypes in estimations of post-partum anoestrus interval based on fortnightly measures of blood progesterone post-calving. There were no differences between experimental Genotypes in the production measures of days-to-calving or birth, growth and weaning weights but on the low-nutrition treatment days-to-calving increased, and growth and weaning weights were lower than on the high-nutrition treatment. Although not statistically significant, there was a strong trend indicating that low-NFI animals consumed fewer megajoules of energy per kilogram of beef weaned. Nutritional treatment did not affect one particular Genotype more than the other but in all Genotypes, animals on low-nutrition consumed fewer megajoules of energy for each kilogram of calf liveweight (beef) weaned. This result suggests that selection for increased feed-efficiency will enable producers to increase stocking rates and that restricted nutrition will not decrease productivity. Blood parameters were measured before and after calving to determine whether the Genotypes were associated with different physiological responses to nutritional restriction. No single blood parameter could be used as a marker to distinguish one Genotype from another. Beta-hydroxybutyrate and leptin were most closely associated with body condition and energy balance and differed between Genotypes when there was a difference in adiposity. Mutations in the bovine leptin gene were examined to determine whether associations with fatness and feed-efficiency, previously reported in North American cattle, were evident in Australian cattle. The mutations were found to exist in the experimental cattle but with differing distributions and associations to those previously reported. An association between one polymorphism (E2JW) and feed-efficiency was noted but shown to be inappropriate for use as a tool in marker-assisted selection. Other associations with circulating leptin concentrations were reported. The studies reported in this thesis showed that after two breeding cycles, Bos taurus cattle selected for reduced fatness or increased feed-efficiency were not compromised in terms of Maternal Productivity when nutrition is restricted. Producers can be re-assured that continued selection for these desirable traits will not impact in a negative way on the female herd. However, it must be noted that the experiment will continue for another three generations and consequently the results might change.

Maternal Productivity in Beef Cattle: The impact on the female herd of genetic selection for a divergence in fatness or feed efficiency

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