Long-term effect of nutrition on the metabolic status and reproductive potential of Merino rams under grazing conditions

Abstract

To evaluate the impact of feeding rams for 14 weeks with legumes and supplement on their clinical, behavioural and endocrine responses, 32 Merino rams, 17 months old, were allocated to four groups (n = 8). The 2 × 2 factorial arrangement evaluated the pasture type (IP, improved pasture; NP, native pasture) and the effect of feeding (+S) or not (–S) feeding a supplement. Every 2 weeks liveweight, scrotal circumference and serving capacity were evaluated and blood samples taken to measure the concentrations of metabolites and metabolic hormones. Grazing behaviour was evaluated every 4 weeks. Rams grazing IP (61.8 ± 0.7 kg) were heavier than those grazing NP (59.6 ± 0.7 kg; P < 0.05), and +S rams (61.8 ± 0.7 kg) heavier than –S rams (59.3 ± 0.6 kg; P < 0.05). Compared with rams from the IP+S, IP–S and NP+S, rams grazing NP–S lost weight (P < 0.001) during the experiment. The scrotal circumference was larger in IP+S (30 ± 0.4 cm), IP–S (30 ± 0.4 cm) and NP+S (31 ± 04 cm) than in NP–S (29 ± 0.4 cm; P < 0.001). Only in rams from the group NP+S was a significant increase in scrotal circumference observed during the trial (P < 0.05). The supplement tended to maintain the mating potential elevated towards the end of the experiment (+S: 74.3 ± 5.4 and –S: 61.7 ± 6.3 ewes/ram; P = 0.07). Supplemented rams were less frequently seen grazing (47 ± 3%) than non-supplemented rams (57 ± 4%; P = 0.09). Rams grazing IP ruminated more frequently (13 ± 2%) than those grazing NP (9 ± 2%; P < 0.001). Events such as lying down, playing, self-grooming and drinking water were more frequent in supplemented (36 ± 2%) than non-supplemented rams (30 ± 2%; P = 0.09). The supplement increased the concentrations of insulin and glucose and decreased the concentrations of non-esterified fatty acids, albumin and urea (P < 0.05). IP increased the concentrations of insulin-like growth factor-I, protein, albumin and urea and tended to increase the concentrations of insulin. We conclude that feeding Merino rams a high plane of nutrition reduced the frequency of grazing events and stimulated a long-term improvement of the metabolic status, evidenced by an increase in liveweight and scrotal circumference, with relevant implications for their mating potential.