ASSOCIATIONS BETWEEN INDIVIDUAL COW FACTORS AND MILK-PROTEIN PRODUCTION

Associations between stage of lactation, cow characteristics, and prot ein production were evaluated using data from a 2-year period on 75 On tario, 5 Alberta, and 3 Nova Scotia dairy farms. Individual-cow protei n production was defined by 305-day protein yield and by the estimated breeding value for protein yield. Lactation curves for average daily protein yield were computed by parity, breed, and season of calving. M ean protein yield was highest in early lactation. However, there was n o pronounced peak in daily protein yield. Parity was positively associ ated with 305-day protein yield and negatively associated with the est imated breeding values for protein yield. First-calf heifers had lower protein yields in early lactation and a slower rate of decline in pro tein yield in late lactation, as compared to later parity cows. Holste in cows had higher unadjusted protein yields and lower protein yields after adjusting for milk yield than other breeds. Holstein cows had si gnificantly higher protein yields early in lactation compared to other breeds, but the rate of decline in protein production in late lactati on was also greater. Season was associated with 305-day protein yield; the highest protein yields occurred in cows calving in the fall and w inter months, but these cows had the greatest rate of decline in prote in production in late lactation. (C) 1998 Elsevier Science B.V.