Abstract
This study was aimed to determine whether the presence of sexually stimulated rams by photoperiodic and melatonin treatments can advance the resumption of post-partum sexual activity in Mediterranean ewes lambing in January and weaned at the end of the breeding season at 41°N, in March. Rams were exposed to two months of long days (16 h light/day) and given three melatonin implants at the end of the long days (sexually-activated rams; SAR). Control rams (CR) were exposed to the natural photoperiod. Thirty-six ewes weaned on 25 February were assigned to one of two groups. From 1 March to 30 June, one group was housed with four SAR males (SAR-treated; n = 18), and the other group (CR-treated; n = 18) was housed with four unstimulated rams. Ovulation was assessed once per week based on plasma progesterone concentrations. Estrus was monitored daily by marks left on ewes by rams’ harnesses. SAR-treated ewes had a shorter (p < 0.01) weaning–first estrus interval than CR-treated ewes (61 ± 17 days vs. 102 ± 47 days; mean date of first estrus after weaning on April 26 and June 6, respectively). The proportion of the ewes ovulating in April or May was higher (p < 0.05) in the SAR-treated group than in the CR-treated group. SAR-treated ewes resumed estrous activity sooner than CR-treated ewes such that, in April, May, and June, the proportion of females that exhibited estrus was higher (p < 0.01) in the SAR-treated group (72%, 89%, and 100%, respectively) than in the CR-treated group (17%, 44%, and 61%, respectively). In conclusion, the introduction at weaning of sexually activated rams advanced the resumption of estrous activity in ewes in spring. The practical implications of this work could be important in ewes adapted for intensive production and accelerated lambing systems.
Highlights
The photoperiod is the main environmental factor that influences the seasonality of reproduction in sheep [1]; other natural environmental factors such as nutrition, temperature, and the presence of conspecifics can have a modest influence on the timing of the onset and the offset of seasonal reproduction [2,3,4]
We demonstrated that ewes continuously exposed to sexually active rams that had been stimulated by an artificial photoperiod and melatonin implants maintained their ovarian activity in spring, which resulted in an increase in estrous expression [19]
We have demonstrated that this combination of artificial photoperiod and melatonin treatment stimulates testosterone secretion in Rasa Aragonesa rams in spring, differing significantly between the treated and the control rams [20]
Summary
The photoperiod is the main environmental factor that influences the seasonality of reproduction in sheep [1]; other natural environmental factors such as nutrition, temperature, and the presence of conspecifics can have a modest influence on the timing of the onset and the offset of seasonal reproduction [2,3,4]. In Mediterranean sheep breeds, the onset is earlier and the offset of the breeding season is later than they are in breeds that live at more northern latitudes [6]; Mediterranean breeds have a shorter seasonal anestrus (i.e., three months from April through June in the Rasa Aragonesa breed [7]) than do Northern European breeds (i.e., 7 months from March through September in the Scottish Blackface breed [8]) Those females respond better to management practices such as the ram effect or the strategic use of feed supplements, which shorten the periods of reproductive inactivity [9]
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