Effect of genomic inbreeding on lactation yields of milk, fat and protein and fertility of grazing dairy cows in two New Zealand farms : a thesis presented in partial fulfilment of the requirements for the degree of Master of Animal Science at Massey University, Palmerston North, New Zealand

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Massey University
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Inbreeding depression is the reduction in phenotypic performance associated with inbred animals. The objective of this study was to calculate the effect of genomic inbreeding on milk production (milk, fat, protein yields and somatic cell score) and fertility traits (start of mating to first service and 21-day submission rate). Genomic inbreeding was calculated through genomic relationship matrices with a panel of 41,417 single nucleotide polymorphisms from 1,708 Holstein-Friesians (F), 412 Jersey (J) and 2,895 Holstein-Friesian × Jersey (F×J) cows. Lactation records and mating records were taken from the 2015 to 2020 production seasons of two New Zealand farms. The average inbreeding Jersey cows had higher levels of inbreeding than F and F×J cows. The effect of inbreeding was most detrimental on milk yield (-57 L/cow per 1% of inbreeding), but also decreased fat and protein yield (-1.7 kg/cow and -1.7 kg/cow per 1% of inbreeding). However, there was no significant effect of inbreeding on somatic cell score and reproductive performance traits. Further analysis with a larger study population and use of other methods to calculate genomic inbreeding coefficients are recommended for future studies.