Sexual behaviour in sheep is driven by body condition and the related nutritional and metabolic status

To evaluate the effects of nutrition on the sexual behaviour of ewes, forty-eight mature (7.6 ± 1.9 yearsold) Merino d’Arles ewes with an average initial body condition score (BCS = 2.26 ± 0.23) and similar bodyweight (BW = 46.6 ± 3.8 kg) were fed three contrasting diets for three months. The diets provided vary-ing levels of energy expressed as proportion of Maintenance Energy Requirements (i.e. Low = 70%MER;Medium = 113%MER and High = 180%MER) with the ultimate objective of obtaining different BW andBCS. Blood samples were taken before and at the end of the three-month experimental period to mea-sure plasma metabolites (glucose, GLU; beta-Hydroxybutyrate, OHB; non-esterified fatty acids, NEFA)and hormones (insulin, INS and leptin, LEPT). The ewes’ sexual behaviour, e.g. attractiveness, receptivityand proceptivity, was estimated through normalized pen-tests before and at the end of the experimentafter hormonally-induced oestrus. As expected, the dietary treatments induced significant changes inBW (i.e. the final weights were 38.6 ± 2.5, 47.3 ± 3.6 and 55.0 ± 2.2 kg for Low, Medium and High groups,respectively) and BCS (1.33 ± 0.4, 1.90 ± 0.2 and 2.33 ± 0.2 for Low, Medium and High, respectively). Themetabolic and hormonal changes were in agreement with the known effects of feeding regimes.The sexual behaviours of ewes were only partially changed by feeding regimes. While the scores ofreceptivity (SR) of ewes to males (i.e. number of agonistic interactions with the courting ram) and scoresof proceptivity (SP) towards rams (i.e. time spent close to a pen with rams) remained unchanged (P > 0.05),there were significant (P < 0.001) changes (Low = 2.42; Medium = 3.63 and High = 4.47; P < 0.001) in theirscores of attractiveness (SA: i.e. a higher score is given to the preferred ewes by rams). This SA waswell correlated with the BW (r = +0.83; P < 0.0001) and BCS (r = +0.73; P < 0.0001) of ewes, it was alsonegatively correlated with metabolites (NEFA: r = −0.32; P < 0.05) and positively with anabolic hormones(INS: r = +0.42; P < 0.01). Combining these variables, the SA is clearly explained (R2= 0.79; P < 0.0001;n = 36) by a model including the static (long-term) effects of nutrition (BW and BCS) and the dynamiceffects of nutrition (NEFA and INS). This work demonstrates that the nutritional status of female sheep,during oestrus, produces efficient cues that clearly orient males in their mating choices.