Development of Models Relating Fecal d13C with Dietary d13C for Small Ruminants.

Stable isotopes can be an important research tool to track C and N in grazing experiments. This study tested different proportions of C4 grass and legume and the correlation between dietary δ13C with fecal δ13C. Forty sheep, with average body weight of 20.4 kg, were fed Tifton-85 (Cynodon spp.) and Alfalfa (Medicago sativa) hays, at different levels of substitution: 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% hay of alfalfa hay, composing five treatments in a randomized block design. The trial lasted 45 days, consisting of 40 days for adaptation and 5 days for collection of orts, feed and feces. Fecal samples were collected directly from the rectum, in order to avoid contamination. All samples were collected during the 5-d sampling period, being composite at the end. Feed and fecal samples were dried for 72 hours in an oven (55ºC) and ground in a Willey mill to pass a 2 mm sieve, incubated for 288 hours in situ in order to obtain the digestibility. All collected samples were subjected to indigestible NDF (NDFi) analysis and then determining C, N and their respective stable isotopes in the experimental diets, feces and orts. Samples were analyzed using the dry combustion (Dumas method) in Vario Micro Cube interfaced with a mass spectrometer IRMS Isoprime 100. Average intake of experimental diets 1 through 5 were respectively 57.3, 69.2, 68.9, 77.3, and 72.3 g kg-0.75, respectively. A linear regression between %C3 in the diet and %C3 in the feces resulted in a r2 of 0.94 (Y = 8.66 + 1.04X). The results indicate that fecal δ13C can be used to reconstruct the proportion of C3 and C4 species in binary mixtures.