Effect of protein restriction followed by realimentation on growth, nutrient digestibility, ruminal parameters, and transporter gene expression in lambs

We report the effects of a protein-restricted diet and realimentation with a milk replacer (MR) and starter on the growth, digestibility, fermentation, and expression of the genes involved in volatile fatty acid (VFA) absorption in the lamb ruminal epithelium. Sixteen newborn twin Hu lamb pairs were used. Each pair was fed the MR and starter with a normal protein level (NPL, 250 and 210 g/kg, respectively) or a low protein level (LPL, 190 and 150 g/kg, respectively) from 15 to 60 days of age (restriction period). Then, all lambs were fed the starter containing 210 g crude protein/kg diet from 61 to 90 days of age (realimentation period). Intake was measured daily, and body weight was measured at 15, 60, and 90 days of age. Digestibility trials were carried out from 51 to 60 and from 81 to 90 days of age with four lambs randomly selected from each group. Eight lambs from each group were slaughtered on 60 and 90 days of age before morning feeding, and their ruminal fermentation parameters were measured. The expression levels of the genes involved in VFA absorption in ruminal epitheliums were measured. During the restriction period, feed intake did not differ between the groups (P = 0.116). The final body weight (P = 0.006) and the average daily gain (ADG; P = 0.003) was greater, while the feed conversion ratio (P = 0.011) was lower in the NPL group. No difference was observed in dry matter (P = 0.894) and organic matter (P = 0.495) digestibility, or the N retention/N intake (P = 0.607). The ruminal pH of the lambs fed the LPL diets was greater (P = 0.012). A decrease in ammonia content (P = 0.023) and the total VFA concentration (P < 0.001) was found in the LPL lambs. The molecular acetate content was greater in the LPL group (P = 0.002), but that of propionate was lower (P = 0.034). Protein restriction decreased the mRNA abundance of the transporters, anion exchanger 2 (P = 0.019) and was downregulated in adenoma (P = 0.025), Na+/H+ exchanger 1 (P = 0.017), 2 (P = 0.025), and 3 (P = 0.037). After realimentation, body weight (P = 0.006) and ADG (P = 0.013) were lower in the lambs fed the LPL diet. The feed conversion ratio (P = 0.301) was the same in the two groups. Lambs fed a LPL diet had a greater N retention/N intake (P = 0.031). No difference was determined in the ruminal pH or the total VFA concentration between the groups (P = 0.572). A greater mRNA level of monocarboxylic acid transporter 4 (P = 0.021) and Na+/H+ exchanger 2 (P = 0.049) was observed in the LPL group. These results suggest that realimentation partially recovers nutrient requirements and ruminal function, but lambs subjected to protein restriction still physically lag behind those continuously fed the NPL diet, indicating a permanent effect on later growth owing to nutrient restriction during very early life.