Abstract
Our objective was to determine the effects of feeding oscillating compared with static dietary crude protein (CP) concentrations on nitrogen (N) retention and urea flux across ruminal epithelia. Twenty-seven Suffolk wether lambs (n = 9) were assigned to a medium-CP diet [MEDIUM; 127 g CP·kg dry matter (DM)-1] or to diets with oscillating CP content (OSC) fed in 2 different sequences, i.e. 2 d of low CP (103 g CP·kg DM-1) followed by 2 d of high CP (161 g CP·kg DM-1; OSC-HIGH) or vice versa (OSC-LOW). Diet adaptation was for 24 d, followed by 8 d of total urine and feces collection. On d 33, lambs were slaughtered 4 h after the morning feeding, such that those receiving OSC-LOW and OSC-HIGH diets were slaughtered on d 3 of receiving the low- or high-CP diets, respectively. Ruminal epithelia were collected and mounted in Ussing chambers and the serosal-to-mucosal urea flux (Jsm-urea) was measured using 14C-urea. Ruminal NH3-N concentration was lower (P = 0.001) in lambs fed OSC-LOW compared with those fed OSC-HIGH. Although N intake was similar, retained N (P = 0.001) and microbial N supply (P = 0.001) were greater in lambs fed OSC compared with those fed MEDIUM. The total Jsm-urea was higher (P = 0.001) in lambs fed OSC-LOW compared with those fed OSC-HIGH. Across diets, the addition of phloretin [a known specific inhibitor of facilitative urea transporter (UT)-B] reduced Jsm-urea by 19.5–22.3% (P = 0.001); however, phloretin-insensitive Jsm-urea was the predominant route for transepithelial urea transfer. Taken together, these data indicate that feeding oscillating dietary CP concentrations improves N retention partly by increasing urea recycling to the rumen when animals are fed low-CP diets, but the greater rates of urea transfer cannot be attributable to upregulation of UT-B.
Highlights
In ruminants, the conversion of dietary nitrogen (N) into edible protein products such as meat and milk is very low (20– 30%), with the majority of dietary N (70–80%) being excreted in feces and urine [1,2]
The dietary treatments consisted of a static dietary crude protein (CP) concentration [MEDIUM; 127 g CP×kg dry matter (DM)21] or 1 of 2 regimens where dietary CP was oscillated between a low-CP (103 g CP×kg DM21) and a high-CP (161 g CP×kg DM21) diet on a 48-h basis (OSC-HIGH; i.e. lambs received the high-CP diet for 2 consecutive days followed by the low-CP diet for the 2 d, such that these lambs were receiving the high-CP diet when they were killed on d 33) or vice versa (OSC-LOW)
DM intake tended to be higher (P = 0.06) in lambs fed MEDIUM compared with those fed oscillating CP content (OSC) diets, but it did not differ between lambs fed OSC-LOW and OSC-HIGH (Table 2)
Summary
The conversion of dietary nitrogen (N) into edible protein products such as meat and milk is very low (20– 30%), with the majority of dietary N (70–80%) being excreted in feces and urine [1,2]. In an effort to improve environmental stewardship associated with intensive ruminant production, there is interest in adopting feeding practices that enhance N efficiency. Several studies have demonstrated that feeding diets with oscillating crude protein (CP) concentrations on a 2-d basis (e.g. feeding a low-CP diet for 2 d followed by feeding a high-CP diet for 2 d) can enhance N retention in growing sheep [3,4] and finishing cattle [5,6].
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