The InCalf Project included a large, prospective, observational field study that described the reproductive performance of 29,462 dairy cows in 168 herds throughout Australia. Milk protein percentage was identified as one of six factors associated with herd reproductive performance. In this further study, a subset of the original database was used and then refined to only include Holstein cows with between one and five milk production records during the first 120 days of lactation in seasonal-calving herds which carried out artificial insemination (AI) for at least the first six weeks of the mating period. This subset then comprised 8,795 cows in 66 herds. Cows in the subset were divided into four quartiles based on early lactation milk protein percentage and five intervals from calving to mating start date (MSD). Three week submission rate and six-week in-calf rate were lower (P<0.001) and the 21-week not-in-calf rate was higher (P<0.001) for cows in the lowest compared to the highest quartiles for milk protein percent. The association between milk protein percentage and reproductive performance was driven mainly by the incidence of non-cycling cows with consequent effects on submission rate in the first three weeks of AI as well as in the second three weeks. In conclusion, late calving cows with low milk protein percentage were at the greatest risk of not being submitted for AI and remaining not in-calf at the end of a relatively long mating period of 21 weeks.
Proceedings of the New Zealand Society of Animal Production, Volume 63, Queenstown, 82-86, 2003
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