Short-day and melatonin effects on milking parameters, prolactin profiles and growth-hormone secretion in lactating sheep

We tested the hypothesis that elevated levels of melatonin in lactating sheep, simulated either by short-day conditions or by melatonin implants, would decrease milk yield while disturbing the rhythm of prolactin (PRL) and growth-hormone (GH) secretion. Eight weeks after lambing, lambs were weaned, and the ewes were subsequently used for milking. The ewes were divided into three groups: (i) long-day control group (LDC, n = 20); (ii) longday melatonin-treated group (LDM, n = 20) and (iii) artificial short-day group (ASD, n = 20). Milk yields were recorded individually at 10-day intervals from day 57 of lactation. Blood samples were drawn from the sheep every 28 days to monitor the changes in PRL and GH secretion. On each sampling date, blood samples were collected at 20-min intervals over a 6-h period beginning at 12:00 p.m. The plasma concentrations of PRL and GH were measured using radioimmunoassay. The longest duration of lactation was found in LDC sheep, and the shortest duration was found in LDM and ASD sheep (p < 0.05). The highest milk yield was found in the LDC group. Significantly (p ≤ 0.01) less milk was obtained from the ASD group. The highest PRL concentration was found during the long-day period (from May to July) in LDC sheep. Prolactin concentration decreased with increasing duration of lactation and decreasing day length. The use of melatonin implants (LDM group) or artificial shortday conditions (ASD group) significantly (p ≤ 0.01) decreased PRL secretion and reduced milk yield as early as the beginning of milking. Growth-hormone concentrations were significantly (p ≤ 0.001) higher in LDC ewes than in LDM and ASD ewes. In contrast to the temporal profile of PRL secretion, GH concentrations decreased during the long-day period but increased as lactation progressed and day length shortened. Despite the considerable reduction in GH concentrations, the GH-secretion profile retained its characteristic seasonal rhythm. Therefore, we conclude that exogenous melatonin (from implants or induced by artificial photoperiod manipulation) decreases milk yield primarily through its effect on PRL concentrations in lactating ewes.