True digestibility of protein and amino acids in goats using plants naturally enriched in 13C as a label to determine endogenous amino acid excretion

• The 13C abundance in the amino acid fraction of blood in goats approached to the 13C abundance in C3 or C4 diet with the feeding time extension, and it reached a stable level after labeled for 12 weeks.
• The true protein digestibility in the forestomach, whole intestine and whole digestive tract for goats fed C4 diet tended to be higher than for those fed C3 diet.
• The goat tissue protein could be stable labeled with 13C naturally enriched in the diet, and 13C as natural isotope marker can be used to determine true amino acid digestibility.

The objective of this study was to determine whether 13C labeling occurs in goats fed diets with different natural levels of 13C enrichment and, if so, to use 13C as a label to determine true amino acid (AA) digestibility. Thirty-six Xiangdong black goats with initial body weights of 7.5 ± 0.5 kg were selected as experimental animals and fed dietary treatments consisting of wheat, barley, and soybeans (C3 diet) or corn, sorghum grain, and sugar cane (C4 diet). The 13C abundance of the amino acid fraction (AAF) of the C3 and C4 diets had average delta 13C values of −28.57‰ and −12.93‰, respectively. Three goats/treatment were slaughtered on days 1, 28, 56 and 84 during the labeling phase, and the AAFs of organs were analyzed for 13C abundance. 13C in the blood AAF increased (−21.70‰, −22.65‰, −23.59‰, and −24.43‰, respectively) with increasing feeding durations in goats fed the C3 diet and decreased (−21.13‰, −16.01‰, −13.78‰, and −12.03‰, respectively) in goats fed the C4 diet. Longissimus dorsi, liver and wool AAFs showed similar trends to those observed in the blood. Each tissue showed a significant dietary treatment effect (P < 0.05) and a dietary treatment × labeling phase interaction (P < 0.01). After the labeling phase (day 85), six goats from each treatment group were placed in metabolism cages and fed at 0700 and 1900. Three of the goats from each treatment group were implanted with the ruminal cannula and duodenal fistulae and given a 14-day recovery period. From days 15 to 21, a total of 4 g chromic oxide, used as an indigestible marker, was administered daily via the ruminal fistulae. At 1900 on day 18 of the collection phase, all goats were switched to the opposite diet type. Thereafter, the duodenal fluids and total feces were collected from days 19 to 21. Additionally, one goat that had not been implanted with a ruminal cannula and duodenal fistulae from each treatment group was killed at 0700 (before feeding) on days 19, 20 and 21, and the AAFs of organs were analyzed for 13C abundance. 13C abundance of the blood AAF was used as an index of endogenous protein labeling. Apparent and true protein digestibilities in the forestomach, whole intestine and whole digestive tract were not different (P > 0.05) between the diet treatment groups. The levels of true protein digestibility in the forestomach, whole intestine and whole digestive tract in goats fed the C4 diet tended to be higher than for those in goats fed the C3 diet. On average, the true digestibilities of protein, essential AAs and nonessential AAs were 5.30%, 4.28% 3.17% (forestomach), 5.20%, 5.39% 3.75% (whole intestine) and 3.01%, 4.97%, 4.17% (whole digestive tract) higher, respectively, than the apparent digestibilities of these components in goats fed the C3 diet. In summary, goat tissue protein was labeled with 13C using different diets, and the results show that 13C can be used to determine true AA digestibility.