Changes in Growth Hormone and Prolactin Secretions in Ewes Used for Milk under Different Photoperiodic Conditions

The aim of the present experiment was to determine the growth hormone (GH) and prolactin (PRL) secretions in relation to the milk yield of lambed sheep used for milk under different photoperiodic conditions. The study was carried out at the Experimental Station of the Department of Sheep and Goat Breeding of the Agricultural University in Krakow, using 60 Polish Long-wool sheep assigned to three groups. Sheep, which had lambed in January in group I (n=20) and in June in group II (n=20) were exposed to natural changes in the length of the photoperiod. In group III (n=20), sheep were lambed in June and kept under a long photoperiod (16L:8D) until the end of lactation. In the group I, the highest milk yield (0.43 ±0.08 l) was found in the first month of milking (May). Growth hormones and PRL concentrations in the blood were also the highest (5.49 ±4.2 and 312.6 ±24.3 ng/mL, respectively). In the group III, as in the group II, the milk yield consistently decreased from August to November. The highest GH concentration was found in August (18.22 ±1.3 ng/mL) and the lowest in October (4.1 ±0.6 ng/mL) (P<0.05). The results show that the GH secretion in sheep entering lactation during the shortening photoperiod is higher than in lambed sheep used for milk during the lengthening photoperiod.