Milk yield, prolactin and IGF-I in cows that calve in spring or autumn

Abstract

The majority of New Zealand dairy cows calve in spring so that the greatest feed demand coincides with highest pasture accumulation rates (Garcia and Holmes 1999). Cows that calve in autumn and produce milk in winter, generally achieve lower peak daily milk yields than spring-calving cows (Garcia and Holmes 1999; Suksombat et al. 1994). This can be only partly explained by nutritional effects. Photoperiod effects on circulating IGF-I and prolactin may also affect peak milk production of autumn calving cows (Dahl et al. 2000). The present study was designed to measure concentrations of circulating IGF-I and prolactin at peak lactation of spring- and autumn-calving cows fed on pasture alone or pasture supplemented with maize silage.