Does microbial contamination affect in situ estimation of crude protein degradability of concentrate feedstuffs?

Abstract Text: Microbial contamination (MC) is an important source of errors in in situ methods, thereby resulting in underestimation of crude protein (CP) degradability as well as overestimation of rumen undegradable protein (RUP) content. The aim of this study was to use ¹⁵N to estimate the MC of fractions soluble (a), potentially degradable (b), and the rate of digestion of the fraction b (kd) of CP, as well as to estimate the necessary incubation time to estimate the rumen degradable protein (RDP) of energy and protein feeds considering two outflow rates (0.05 h^-1 and 0.08 h^-1). Twelve concentrates were evaluated. Six energy: wheat bran, rice meal, ground corn, ground sorghum, ground corn cob, and soybean hulls, and six protein: cottonseed meal 38% CP, soybean meal, ground bean, peanut meal, sunflower meal, and corn gluten meal. The feeds were divided into 4 groups and they were ruminally incubated in 4 crossbred bulls for 0, 2, 4, 8, 16, 24, 48, and 72 h. To estimate the MC of the incubated residues, ruminal bacteria were labeled with ¹⁵N via continuous intraruminal infusion of ¹⁵(NH₄)₂SO₄. Ruminal digesta was collected for the isolation of bacteria before the first infusion of ¹⁵N and after the infusion of ¹⁵N during the collection period. There was no difference (P = 0.74) in the parameters a, b, and kd, corrected and uncorrected for MC. All the feeds followed an exponential model of degradation and the model fitted well to the data, except for corn gluten meal, probably, the maximum incubation time used here (72 h) was not long enough to allow an accurate estimation of degradation profile. The cluster analysis allowed (R² = 0.944) grouping feeds into three different groups according to the necessary incubation time to estimate RDP. The first was formed by the high-starch energy concentrates (15.4 ± 0.46 h), the second by the low-starch energy concentrates (6.8 ± 0.60 h), and the third by the protein concentrates (9.9 ± 0.41), considering kp 0.05h^-1. In conclusion, the MC was low and non-significant, so correction of ruminal protein degradation is irrelevant for the concentrate studied. However, the chemical composition of this feeds resulted in different incubation times to estimate RDP content, and it needs to be considered in techniques used to estimate CP digestibility in the rumen and intestines.

Keywords: microbial contamination, protein, rumen degradable protein