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