Abstract :
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.
