The use of high genetic merit Angus and Hereford bulls in a New Zealand dairy herd : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Animal Science, Massey University, Turitea, Palmerston North, New Zealand

Calves born to dairy cows that are not required as replacements for the dairy herd can be sold for beef production. The value of these calves depends on their potential for meat production, consequently, beef-cross-dairy calves are generally more valuable than dairy breed calves. Angus and Hereford are the predominant beef-breed breeds in New Zealand, with Hereford bulls more commonly used in the dairy industry than Angus. The key concern of dairy farmers when choosing a service bull to generate calves for sale for beef production is the health and production of the dairy cow or heifer, requiring a focus on calving difficulty where birth weight of the calf is a contributing factor. The gestation length of the calf impacts the calving interval and calving spread and therefore days in milk of the cow. Of most importance to a beef producer, is how the calf will perform in a beef rearing system. Limited research exists regarding which beef-breed bulls are most appropriate for mating to dairy cows and heifers, and whether there are negative impacts of the bull on milk production and rebreeding of the cow or heifer, which should be considered alongside a change in calf value. The general objective of this thesis was to identify what type, in terms of estimated breeding value (EBV) or genetic merit, of Angus and Hereford bulls are appropriate for mating to dairy cows and heifers. Data from 980 singleton beef-cross-dairy calves and 952 artificially bred mixed-aged dairy cows were used to compare 65 Angus and Hereford service sires of differing genetic merit using a progeny test. Mean progeny birth weight, gestation length, age at weaning and pre-weaning ADG differed among sires, while assistance at birth was less than 1%. The relationship between breeding values calculated from the data and published birth weight and gestation length EBV indicate that the published EBV are good predictors of progeny performance in a beef-cross-dairy system. There was no effect of service sire on post-calving live weight, days in milk, milk production or inter-calving interval of mixed-aged cows. Relationships between calf gestation length and the dam’s date of calving and pregnancy rate, and between calf birth weight and survival indicate the potential for an effect of service sire. However, as lighter calves and shorter gestation lengths are preferable for beef-cross-dairy calves, selection of beef-breed service sires with these traits should minimise any negative effects. Results from this experiment indicate that the bulls used in this experiment, and other bulls with similar EBV for birth weight and gestation length would be suitable for mating mixed-aged dairy cows. Data collected from 304 first-calving heifers and their calves allowed the comparison of Angus, Hereford and Jersey breed bulls specifically selected for use over dairy heifers. Calves sired by Angus and Hereford bulls were heavier at birth than Jersey-sired calves, and 3.6%–10% of beef-sired calves were assisted. There was no difference in the body condition score, pre-calving live weight, milk production, pregnancy rate, inter-calving interval, re-calving date and 21-day re-calving rate of heifers mated to the different breeds of bulls. For dairy heifers, the suitability of beef-breed bulls is reliant on minimising calving difficulty, and selection for bulls with lighter birth weight EBV tended to decrease calving assistance. Provided easy calving bulls are used, the similarities in milk production, rebreeding performance and survival provides evidence for the use of beef-breed bulls over dairy heifers. Overall, the results presented in this thesis provide evidence towards the successful use of high genetic merit beef-breed bulls in New Zealand dairy herds, providing a greater value calf, with little or no negative effect on the dairy cow or heifer. For mating to mixed-aged dairy cows, beef-breed bulls which produce comparable calves to the alternative dairy bulls are most appropriate, industry EBV can be used to predict progeny birth weight and gestation length. For mating to dairy heifers, bulls which have very high calving ease and very low birth weight EBV are suitable to minimise the risk of calving difficulty