Presence of photoperiod-melatonin-induced, sexually-activated rams in spring advances puberty in autumn-born ewe lambs

• The effect of sexually-activated rams on puberty of autumn-born ewe-lambs was studied.
• Age at first ovulation was reduced by the presence of sexually-activated rams.
• First estrus was observed at 8.5 months of age in the treated group.
• Control ewe-lambs exhibited first estrus at 10 months of age.
• Puberty of autumn-born ewe-lambs was advanced by the presence of these rams.

The objective of this study was to determine the effect of sexually activated (by specific photoperiodic treatments) vasectomized rams on the onset of puberty in autumn-born ewe lambs in spring. Photoperiodic-treated rams were assigned to one of three groups, which were sequentially exposed to two months of long days (16 h light/d) and given three melatonin implants at the end of the long days (sexually-activated-rams; SAR): SAR1 (n = 5; 1 December–31 January), SAR2 (n = 5; 1 Jan–28 February), and SAR3 (n = 5; 1 February–31 March). Control rams (CR; n = 4) were exposed to the natural photoperiod. On 1 March, 50 ewe lambs born in September were assigned to the SAR rams (SAR-treated; n = 25) or the CR rams (CR-treated; n = 25). SAR-treated ewe lambs were housed with SAR1 rams from 1 March. SAR2 rams replaced SAR1 rams (14 April), which were replaced by SAR3 rams (22 May) until 30 June. CR-treated ewe lambs were housed with the unstimulated rams. Ovulation was identified by weekly plasma progesterone concentrations and estrous behavior, as indicated by colored rumps. A greater proportion of SAR than CR ewe lambs ovulated in April (52% vs. 0%) and May (68% vs. 0%) (P < 0.0001), and were in estrus in May (64% vs. 0%) and June (92% vs. 24%) (P < 0.0001). The presence of SAR rams reduced (P < 0.0001) mean (±SD) age (d) at first ovulation and estrus (235 ± 28 [7 May] and 257 ± 24 [29 May], respectively), compared to that of CR-treated ewe lambs (277 ± 5 [18 June] and 302 ± 16 [14 July], respectively). In conclusion, the presence of photoperiod-melatonin-induced, sexually activated rams in spring, advanced puberty in autumn-born ewe lambs. Using this technique might provide an effective and sustainable means of increasing the productive life of ewes, while avoiding the use of hormonal treatments.