EFFECT OF PHOTOPERIOD ON MILK-YIELD, MILK -COMPOSITION AND VOLUNTARY FOOD-INTAKE IN LACTATING DAIRY EWES

Effect of photoperiod on milk yield, milk composition and voluntary fo od intake in lactating dairy ewes. Thirty-eight dairy ewes of the Sard a breed were divided in two groups which were equilibrated for zootech nical criteria. They were subjected to artificially controlled photope riods of either long daylight (JL: 15h30 L, 8h30 D) or short daylight (JC: 8h30 L, 15h30 D). The experimental periods began 25 days before l ambing and were maintained until day 150 of lactation. At I-his time t he photoperiod was abruptly changed to equilibrated daylenght treatmen ts (12h L, 12h D). Throughout the whole experiment, the ewes were fed alfalfa hay ad libitum as well as a constant proportion of a commercia l concentrate (table I). JL ewes produced more milk (+25.3%) than JC e wes (fig 3). After 150 days, the daylight increase (+3h30/exJC) made i t possible to maintain milk yield at the initial value (only a -8.0% d rop between day 145 and day 187), while the daylight decrease (-3h30/e xJL) resulted in a sharp drop in milk yield (-37.7%) over the same per iod. The milk solid concentration from the JL ewes (fig 4) was less th an that of the JC ewes. The maximum differences, which occurred betwee n days 30 to 40, were 14.2 g/litre for fat content and 11.0 g/litre fo r protein content. The rapid increase of daylength (exJC) was less fav ourable to milk solid concentration than its decrease (exJL) for both the fat content (-7.8 g/litre) and protein content (-6.5 g/litre). The levels of voluntary food intake (fig 1), which were identical at the end of pregnancy, started bring different on day 25 of lactation. Betw een days 28 and 150 the mean total DM consumption for the JL ewes was 16.3% higher than those of the JC ewes. After the daylength change, th e intake differences were reduced and became stabilised at a mean valu e of +6.2% for the exJL ewes. By the end of experiment, when the photo period had been the same for 30 days, the total food intake levels wer e very similar. Time scale effects of daylight in lactating ewes act p rimarily on milk yield, then on milk composition and finally on food i ntake. Refractoriness to photoperiod seems to appear after being subje cted to constant light treatments for more than 100 days.