Submitted to: Domestic Animal Endocrinology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 5/23/1997
Publication Date: N/A
Citation: N/A Interpretive Summary: Farmers are challenged with the responsibility for maintaining a constant supply of high quality milk but at reasonable price. Public concern for the healthful character of the food consumed has focussed attention on the need to maintain what is perceived as a safe supply of the product. The present research confirmed that the totally benign practice of changing, lengthening, that is, the duration of light that lactating dairy cattle are exposed to increases milk production compared to allowing the duration of light to follow a natural seasonal daylength. This is particularly important during the winter months when daylength can decrease to as low as 12 hours. Simply by artificially maintaining 18 hours of light in the housing barn, milk production during January through April was shown to increase. We have discovered a possible natural endocrine hormone reason for this increase in milk production. Long day photoperiod increases blood concentrations of the hormone insulin-like growth factor-I, an endogenous hormone known to regulate milk production in all mammalian species. More work is required to determine how far milk production can be increased by this technique, but the method offers promise of good return with no carryover of performance enhancing compounds to the food chain.
Technical Abstract: Relative to short days, exposure to long day photoperiod (LPDD) is known to increase milk yield in dairy cattle. The endocrine basis for this phenomenon, however, is unknown. The present study was designed to test the hypothesis that LDPP was associated with increased circulating IGF-I, a hormone previously shown to be galactopoietic in ruminants. Forty lactating cows were exposed to either ambient daily photoperiod (< 13 hrs light/d; ADPP) or to a constant light treatment of 18L:6D (LDPP) between January and April, 1995. Animals were fed to appetite a ration formulated to meet the nutritional requirements of lactation. Milk yield and dry matter intake were quantified daily and blood samples were collected via tail vein every 14 d. Plasma was harvested and assayed for IGF-I. LDPP increased milk yield relative to ADPP, the increase became significant (P<0.02) afterr 28 d of treatment and was maintained for the duration of the study. In addition, LDPP cows had greater circulating IGF-I than animals exposed to ambient photoperiodic conditions. IGF binding proteins -2 and -3 were not different between groups. These results support the hypothesis that photoperiod influences circulating IGF-I in lactating cattle and provides a possible endocrine mechanism for the galactopoietic response to LDPP.