Tracking in Situ Ruminal Degradation of Grasses and Legumes Using d13C.

Warm-season C4 grasses and legumes have different C isotopic composition and that might help to track the C sources during ruminal degradation. The objective of this study was to evaluate forage ruminal decomposition by monitoring the disappearance of different carbon sources (C3 or C4) using C stable isotope ( δ13C ). Diets tested were : 1) 100% Tifton-85 hay; 2) 75% Tifton-85 + 25% alfalfa hay; 3) 50% Tifton-85 + 50% alfalfa hay; 4) 75% alfalfa + 25% Tifton-85 hay; 5) 100% alfalfa hay, composing five treatments in a randomized block design. The in situ degradability was performed to obtain the degradation curve of each diet. Samples of the experimental diets were weighed and placed into nylon bags into a rumen fistulated cow. Incubation times were 0, 3, 6, 12, 24, 48, 72, 96, 144, and 288 hours. Incubation started with the last incubation time. After incubation, the bags were withdrawn, washed in water and dried in an air-forced oven (55°C) for 72 hours. All collected samples were subjected to indigestible NDF (iNDF) analysis. After iNDF analyzes, samples were analyzed for δ13C, total C, δ15N and total N. Analyzes were performed using the dry combustion (Dumas method) in Vario Micro Cube interfaced with a mass spectrometer IRMS Isoprime 100. Diet and fecal δ13C, as well as δ13C in the diet and fecal iNDF were correlated to verify the relationship between them. Fractionation occurred during the digestive process, with fecal samples more depleted than dietary samples (-30.64 vs. -32.30‰), however, when iNDF samples were analyzed, the fractionation was reduced (-31.67 vs. -32.06‰) in alfalfa diets. Based on the results from the alfalfa diet, fecal iNDF δ13C can be used to reconstruct the diets of binary mixtures (C3 and C4) in ruminants reducing the fractionation between dietary and fecal C.