Abstract

Continuously Changing Light-Dark Phase Decreases Milk Yield, Fat, Protein and Lactose in Dairy Cows

 Photoperiod impacts feeding behavior, health, growth and milk production of dairy cattle. In humans behaviors that are asynchronous with the natural light-dark cycle, such as shift work or jet lag, are strongly associated with metabolic disorders as well as impaired reproductive performance. The objective of this study was to determine the effects of exposing mid-lactation dairy cows to chronically shifting 8h light (L)-8h dark (D) cycles on feed intake, milk yield, milk composition and mammary gene expression. Six first lactation Holsteins ∼90d in milk were maintained on a 16 h L: 8 h D cycle and milked at 5AM and 4PM for 7d (control, Period 1). During Period 2, 7 d immediately following Period 1, cows were exposed to continuous cycles of 8 h L: 8 h D, but maintained on the same milking and feeding schedule. Exposure to chronic 8 h LD cycles significantly depressed milk yield (P<0.05), but did not affect daily feed intake. Percent milk fat, protein and lactose were not different, but milk urea nitrogen (MUN) significantly increased. On the last day of each period mammary gene expression was measured by Q-PCR of total RNA isolated from the cytosolic components of milk fat globules. Expression of the core clock gene BMAL1 in addition to Beta-casein, Alpha-lactalbumin, Fatty acid synthase and Acetyl CoA-carboxylase were all decreased (P < 0.05) after 7 d of chronic 8 h LD cycles. The results from this study show for the first time that exposing lactating cows to chronic light shifts decreases milk production and may alter metabolism. Further, experimental design may provide a paradigm to study the effects of changing lighting schedules on milk production and a potential model to study effects of disrupting circadian system on production efficiency