Evaluating grazing nutrition of alpine merino growing rams in the regreening and grassy stages of pasture herbage

Alpine Merino growing rams grazing in the Qilian Mountains, the research object in this study, were used to measure forage intake, nutrient digestibility, and rumen parameters in herbage regreening and grassy stages to determine the forage nutrition supply law and the nutritional status of the rams during these stages. The dominance index of plant communities in the grazing grassland and the diversity index, richness index, evenness index, and aboveground biomass of pasture in the grassy stage were higher than those in the regreening stage (P < 0.05). Additionally, the grading index (GI), crude protein, and crude ash were higher in the grassy stage than in the regreening stage (P < 0.05). There was no difference in the dry matter and crude protein intake of Alpine Merino growing rams, but the metabolic energy intake was higher in the herbage grassy stage than in the regreening stage (P < 0.05). In terms of digestibility in the two stages, there was no significant differences between the dry and organic matter (P > 0.05), but the crude protein, neutral detergent fiber, and acid detergent fiber were higher in the regreening stage (P < 0.05). The total volatile acid content was higher in the regreening stage than in the grassy stage (P < 0.05), but the isobutyric acid, butyric acid, isovaleric acid, and ammonia nitrogen content were higher in the grassy stage than in the regreening stage (P < 0.05). In the regreening stage, the metabolizable energy intake and crude protein intake of the Alpine Merino growing rams were 9.32 MJ and 45.23 g·d⁻¹ per sheep lower than the nutritional requirements, respectively, according to the feeding standard of Chinese Merino sheep (body weight 70 kg, daily gain 100 g). In the grassy stage, the metabolizable energy intake and crude protein intake were 7.32 MJ and 32.64 g·d⁻¹ per sheep lower than the nutritional requirements, respectively. In conclusion, the GI value and productivity of pasture in the grassy stage were higher than those in the regreening stage. Furthermore, when the daily gain reached 100 g in 70 kg Alpine Merino sheep, the metabolizable energy intake and crude protein were insufficient, and a certain amount of this energy and protein feed needed to be supplemented.