Milk composition and productive and reproductive performance of cows from A1 and A2 β-casein variants, milked once or twice a day : a thesis presented in partial fulfilment of the requirements for the degree of Master of Science (Animal Science) at Massey University, Palmerston North, New Zealand

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Protein is an important component of milk and it plays an essential role in all living organisms. β-casomorphins-7 (BCM-7) is derived from A1 β-casein and has been implicated in some human health issues. This A1 β-casein is produced by cows with the A1A1 or A1A2 genotype, whilst cows with the A2A2 genotype produce A2 β-casein, which has not been implicated in the same human health issues. Given the potential importance of A2 milk for public health and its apparent commercial potential, selection based on the A2 type and its impact on production and reproductive traits should be investigated. The objective of the current study was to compare the productive and reproductive performance of dairy cows based on A2 type in two different dairy farms. From July 2017 to May 2018, 206 cows (including 122 A2A2 genotype; “A2 cows”) were milked once a day at Dairy 1 and 451 cows (including 217 A2 cows) were milked twice a day at Dairy 4. Records of lactation yields of milk, fat and protein, fat percentage, protein percentage, days from start of mating to conception, pregnancy rate to first service, the submission rate at 21 days and the pregnancy rate at 21 and 42 days (PR42) after the start of mating from 642 cows in two herds were analysed. The effects of A2 type on productive and reproductive traits were not significant. The interaction between farm and β-casein genotype was significant for PR42 (P<0.05) but not for any other traits. The interactions between parity number and genotype were not significant for any of the traits. The results indicated that cows of different β-casein types have similar production and reproduction performance.