High-protein feed elevates ruminal and plasma ammonia and is associated with altered open-field behavior in Suffolk...

Rumen fermentation shapes metabolic and endocrine functions. However, its association with stress behavior in ruminants remains underexplored. Therefore, we evaluated the effects of feed energy and protein levels in physiological and behavioral stress responses by altering rumen production in Suffolk sheep. Seventeen Suffolk sheep were fed high-energy (HE), high-protein (HP), or control feeds. Behavior in the open field test (OFT) was quantified as the latency to enter the field, number of squares entered, frequency of bleats, sniffing, and latency to bleat. Moreover, rumen volatile fatty acid, ammonia, plasma ammonia, cortisol, and aldosterone levels were measured. The HE group was characterized by a lower acetate-to-propionate (A/P) ratio than that in the control. Similarly, HP sheep were characterized by high ruminal and plasma ammonia levels. In the OFT, both the HE and HP groups bleated and sniffed more frequently than the control group; the number of squares entered was greater in the HP group than in the control group. Plasma cortisol and aldosterone levels did not differ according to the feed. Furthermore, plasma ammonia levels correlated with a greater number of squares entered (ρ = 0.78), whereas a lower A/P ratio correlated with lower cortisol levels (ρ = -0.56). Our findings suggest that feed composition can modulate stress responses, particularly stress-related behaviors, in sheep. This study provides preliminary evidence that feed-induced changes in rumen metabolites, particularly ammonia, are associated with behavioral stress responses in sheep. These findings may help inform feeding strategies that consider animal welfare.