Melatonin treatment in spring and reproductive recovery in sheep with different body condition score and age

• Melatonin treatment has been found to influence the reproduction out of season in sheep.
• Different factors are involved in the response to melatonin treatment in sheep.
• We found that the treated animals exhibited a shorter interval from ram introduction to lambing compared to untreated.
• The BCS and age should be kept into account in order to improve the efficiency of melatonin implants.

With the aim to evaluate the effect of melatonin treatment on the advanced reproductive recovery in sheep with different body condition score (BCS) and age, 800 ewes were selected from two farms. These ewes (3–6 years old, multiparous and with BCS 2.5–4.0) were subdivided into two Groups (200 M and 200 C), balanced on their BCS and age. On 20 March, Group M was treated with one melatonin implant (18 mg). Group C was untreated. Males were introduced to the groups 35 days after treatment. Gestation was diagnosed between day 45 and 90 after mating by transabdominal ultrasonography. From day 150 to 190 after rams introduction, lambing date and newborns’ number were recorded. The average time in days between male introduction and lambing resulted shorter in treated than in control ewes (166.4 ± 0.48 vs. 172.5 ± 0.50) (P < 0.05). At day 160 and 170 from ram introduction the fertility rate was higher in Group M than in C (P < 0.05). The overall fertility at day 190 from rams introduction showed no differences between Group M and C (337 and 339, respectively). At day 170 from male introduction the number of the 5–6 years-old lambed ewes were 2-fold higher than the youngers (P < 0.05). The animals with a BCS 3.5–4.0 had a faster response to male effect, and a shorter mean distance in days from rams introduction to lambing, compared to those scored 2.5–3.0 (166.1 ± 0.48 vs. 174.8 ± 0.51) (P < 0.05). We concluded that the ewes with BCS 3.5–4.0 and aged 5–6 years showed a better response to melatonin treatment in spring.