PHOTOPERIODIC REQUIREMENTS FOR RAPID GROWTH IN YOUNG MALE RED DEER

Winter growth of young male red deer can be increased by exposure to 1 6 h of light (L) and 8 h of dark (D) per day (16L : 8D). This study te sted the duration of photoperiod required for this growth response, de termined if the time to reach slaughter weight can be reduced and moni tored plasma IGF-1, prolactin and reproductive development. Fifty male calves were allocated to five equal groups. Four groups were housed i ndoors and for 33 weeks from the winter solstice (22 June, southern he misphere) until 11 February were placed tinder either 16L:8D (16L), 13 .25L:10.75D (13L), 10.75L:13.25D (11L) or 8L:16D (8L) photoperiods. Th e fifth group of deer (OC) remained outside in a gravelled enclosure. All groups were given a pelleted diet ad libitum. Group food intake wa s recorded daily, individual live weight was measured weekly and teste s diameter and blood samples taken at weekly or 2-week intervals. Plas ma prolactin concentrations in 16L increased within 4 weeks of treatme nt and were different (P < 0.001) between groups from 14 August to 4 S eptember. IGF-1 increased in both 16L and 13L 4 weeks after treatments and then increased further in 16L above that of 13L (P < 0.01). All g roups grew at the same rate for the first 7 weeks. 16L then gained mor e weight (P < 0.001) than the other groups over the next 19 weeks (50. 7 kg v. 38.5 for 13L, 35.7 for 11L, 37.0 for 8L and 37.4 for OC; s.e.d . 3.76). Food intake was positively related to growth rate in a simila r way among the inside groups (P < 0.001), however there was a higher energy requirement outdoors (P < 0.05). A target live weight for slaug hter of 95 kg was reached 7 weeks earlier for 16L than the other group s (P < 0.01). Testes diameter of 16L was larger than in the other grou ps from 13 November until 24 December (P < 0.001). The growth of 16L s lowed from 1 January while that of OC increased and the live weight of OC was equal to 16L by the end of the experiment. OC also had the lar gest testes diameter from 5 February onwards (P < 0.01). The live-weig ht increase in OC was associated with increases in both prolactin and IGF-1 levels. This study confirmed that 16L: 8D stimulates rapid growt h of young male red deer during winter for sufficient time to achieve an earlier slaughter date. The live-weight advantage was lost by late summer however. The increased growth rate was mediated by food intake and associated with increases in IGF-1 and prolactin and earlier repro ductive development. Photoperiods of 13 h of light per day or less did not stimulate growth and increases in IGF-1 and prolactin were of a l ower amplitude than under 16L : 8D.