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 food intake in lactating dairy ewes. Thirty-eight dairy ewes of the Sarda breed were divided in two groups which were equilibrated for zootechnical criteria. They were subjected to artificially controlled photoperiods of either long daylight (JL: 15h30 L, 8h30 D) or short daylight (JC: 8h30 L, 15h30 D). The experimental periods began 25 days before lambing and were maintained until day 150 of lactation. At I-his time the photoperiod was abruptly changed to equilibrated daylenght treatments (12h L, 12h D). Throughout the whole experiment, the ewes were fed alfalfa hay ad libitum as well as a constant proportion of a commercial concentrate (table I). JL ewes produced more milk (+25.3%) than JC ewes (fig 3). After 150 days, the daylight increase (+3h30/exJC) made it possible to maintain milk yield at the initial value (only a -8.0% drop between day 145 and day 187), while the daylight decrease (-3h30/exJL) resulted in a sharp drop in milk yield (-37.7%) over the same period. The milk solid concentration from the JL ewes (fig 4) was less than that of the JC ewes. The maximum differences, which occurred between days 30 to 40, were 14.2 g/litre for fat content and 11.0 g/litre for protein content. The rapid increase of daylength (exJC) was less favourable 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. Between 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, the intake differences were reduced and became stabilised at a mean value of +6.2% for the exJL ewes. By the end of experiment, when the photoperiod had been the same for 30 days, the total food intake levels were very similar. Time scale effects of daylight in lactating ewes act primarily on milk yield, then on milk composition and finally on food intake. Refractoriness to photoperiod seems to appear after being subjected to constant light treatments for more than 100 days.