Associations between milk protein percentage and reproductive performance in Holstein-Friesian cows in seasonal-calving dairy herds

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 6 factors which had a moderate to large effect on herd reproductive performance. A subset of the original database was used and then refined to only include Holstein-Friesian cows with between 1 and 5 milk production records during the first 120 days of lactation in seasonal-calving herds which carried out AI for at least the first 6 weeks of the mating period. This subset comprised of 8,795 cows in 66 herds. Cows in the subset were categorised into quartiles based on milk protein percentage (low: 2.36 to 2.99%; med-low: 3.00 to 3.14%; med-high: 3.15 to 3.29%; and high 3.30 to 4.59%) and 5 groups based on interval from calving to mating start date (>12 weeks; 9-12 weeks; 6-9 weeks; 3-6 weeks; ≤3 weeks). Cows with low milk protein percent had a different calving pattern from those with higher milk protein percent. Whereas 92.5% of cows in the high milk protein quartile had calved at least 3 weeks at mating start date, only 84.3% in the low milk protein quartile had calved by the same date. The mean 3-week submission rate and 6-week in-calf rate were lower (P0.05) between quartile of milk protein percent. The association between milk protein percentage and reproductive performance was due, in part, to the prevalence of non-cycling cows with consequent effects on submission rate in the first 3 weeks of AI. In conclusion, late calving cows with low milk protein percentage were at greatest risk of not being submitted for AI in the first 3 weeks of mating and not being in-calf within 6 weeks of the start of matin