Effects of melatonin implants on locomotor activity, body temperature, and growth of lambs fed a concentrate-based diet

Melatonin is involved in the regulation of circadian rhythms and is implicated in seasonal reproduction in sheep. In several physiological mechanisms, it acts as an antioxidant and an anti-inflammatory molecule, regulating energy metabolism. This work investigated the effects of melatonin implants at 30 days of age on locomotor activity (LA), body temperature, and growth of fattening female and male lambs. Sixty lambs were divided into two groups: one of which received melatonin (MEL, 15 males, 16 females) and a control group (CTR, 16 males, 14 females). In the melatonin group, two 18 mg melatonin implants were placed at 30 days of age. Lambs were fattened for 6 weeks from weaning (45 days of age) to slaughter (85 days). The feed conversion rate (FCR) was calculated based on live weight and the amount of concentrate consumed. LA was measured weekly by actigraphy, and circadian rhythmicity was calculated. Rectal (Trec) and surface temperatures (Tsur) were recorded in the last week of fattening, and subcutaneous fat thickness (FT) over the longissimus dorsi muscle was measured by ultrasound scanning. Treatment did not affect FCR, although MEL female lambs consumed significantly (P < 0.001) less concentrate than CTR lambs. Treatment and sex had a significant (P < 0.05) interaction effect on FT; specifically, FT was significantly (P < 0.05) higher in female MEL lambs (3.22 ± 0.21 mm) than in female CTR lambs (2.57 ± 0.24 mm), but FT in males did not differ between MEL (2.77 ± 0.21) and CTR (2.94 ± 0.24 mm) lambs. Overall activity was significantly (P < 0.001) lower in the MEL lambs (72.22 ± 0.10 counts/min) than in the CTR lambs (78.89 ± 0.12 counts/min). MEL lambs had a significantly (P < 0.01) lower Trec (CTR: 39.00 ± 0.07; MEL: 38.68 ± 0.10) and Tsur for all body regions evaluated than the CTR lambs. In conclusion, treatment with exogenous melatonin at 30 days of age increased food efficiency in fattening female lambs, probably, because of the lower metabolism in treated lambs, which was reflected by the lower body temperature and LA exhibited by these animals. In addition, the study has demonstrated the effect of exogenous melatonin in the growth performances of post-weaning lambs, and that its effects depend on animal sex, which suggests that treatments that target females might be most appropriate in the fattening period.