Phenotypic relationships between milk protein percentage and reproductive performance in three strains of Holstein Friesian cows in Ireland

Abstract

The relationship between milk protein percentage and fertility in seasonal calving, dairy cattle in Ireland was quantified using a total of 584 lactation records, collected over a five-year period from experiments comparing three strains of Holstein-Friesian cows under three different feeding systems. Logistic regression analyses showed that increased protein percentage during early lactation was positively associated with the probability of a cow becoming pregnant to its first service (P <0.05). Similarly, protein percentage during the lactation had a positive (P <0.01) association with overall pregnancy rate. The results suggest that negative energy balance in early lactation or during the whole lactation causes a shortage of glucose to the udder, this restricts the synthesis of milk protein in the udder and causes a lower milk protein percentage. During negative energy balance there is also a concurrent reduction of IGF-І, LH and oestradiol secretion, which consequently delay ovarian follicular development, and hence impairs reproductive performance. In conclusion, cows with higher milk protein percentage during early lactation have a greater likelihood of becoming pregnant earlier in the breeding season, and have a higher conception rate.