EFFECT OF NUTRITIONAL GROWTH RESTRICTION ON TIMING OF REPRODUCTIVE DEVELOPMENT AND PLASMA-CONCENTRATIONS OF INSULIN-LIKE GROWTH-FACTOR-I AND GROWTH-HORMONE IN MALE RED DEER (CERVUS-ELAPHUS) REARED IN CONSTANT PHOTOPERIOD

The effect of nutritional growth restriction on reproductive developme nt in red deer stags reared in constant photoperiod was investigated a nd the correlation between reproductive status and circulating concent rations of growth hormone (GH) and insulin-like growth factor-1 (IGF-1 ) examined. Stags were reared from birth in constant photoperiod (12 h light : 12 h dark). They were offered a 'complete diet' ad libitum un til they reached 52.5 kg live weight (LW) and were then maintained at this LW by restricting daily food dry-matter intake (DMI) for either a long (LR, 26 to 51 weeks of age,no. = 5) or short (SR, 21 to 33 weeks , no. = 6) period before being returned to ad libitum feeding. Relativ e to SR stags, antlers hardened later in LR stags (72.0 v. 57.5 weeks of age, P<0.001); sustained elevations in plasma testosterone also beg an later (55.2 v. 38.5 weeks, P<0.001) but at lower LW (54.0 v. 60.6 k g, P<0.05). In both groups , the testosterone rise followed the return to ad libitum food by 4 to 5 weeks. Plasma IGF-1 was closely correlat ed with DMI (LR, r = 0.84, P<0.001; SR, r = 0.93, P<0.001) and with pl asma testosterone (LR, r = 0.42, P<0.001; SR, r = 0.38, P<0.01). Also the increase in plasma IGF-1 preceded that of testosterone (by 2.8 (s. e. 0.94) weeks) and was associated with a transient plasma GH peak (P< 0.05). The timing of reproductive development in growth-restricted mal e red deer in constant photoperiod may therefore be more sensitive to DMI than LW, and changes in plasma IGF-1 concentrations are consistent with a putative permissive role in relaying this information.