In vivo determination of rumen undegradable protein of dried distillers grains with solubles and evaluation of duodenal microbial crude protein flow.

The objectives of this trial were to determine the rumen undegradable protein (RUP) of dried distillers grains with solubles (DDGS), to compare the estimates of duodenal bacterial CP (BCP) flow using diaminopumelic acid (DAPA) or DNA as bacterial markers, and to estimate duodenal protozoal CP (PCP) and yeast CP (YCP) flow when DDGS are fed. Three crossbred steers fitted with ruminal and double L-shaped duodenal cannulae (average BW 780 ± 137 kg) were used in a 3 treatment, 6 period crossover design. Animals were housed in individual free stalls and fed twice daily at 0700 and 1900 h. Diets (DM basis) were 1) CONTROL, which is 0% DDGS but with 19.5% corn bran, 20% sorghum silage, 60% brome hay, 0.5% trace minerals, and 0.25% urea, 2) LOW DDGS, which is inclusion of 9.75% DDGS replacing equal percentage of corn bran, and 3) HIGH DDGS, which is inclusion of 19.5% DDGS completely replacing corn bran. Duodenal BCP flow was estimated using DAPA and DNA as bacterial markers. In addition, duodenal PCP and YCP flow were estimated using DNA markers. The value of DDGS RUP as a percent of CP was determined to be 63.0 ± 0.64%. Estimates of duodenal BCP flow using DAPA were 473, 393, and 357 ± 78 g/d (P = 0.09) for CONTROL, LOW DDGS, and HIGH DDGS, respectively. Estimates of duodenal BCP flow using DNA were 479, 397, and 368 ± 74 g/d (P = 0.14), respectively. Average BCP flow across treatments was unaffected (P = 0.71) by marker type and were 404 and 417 ± 83 g/d for DAPA and DNA markers, respectively. Estimates of duodenal PCP flow were 82, 80, and 78 ± 12 g/d (P = 0.64) for CONTROL, LOW DDGS, and HIGH DDGS, respectively. Estimates of duodenal YCP flow were 0.15, 1.94, and 4.80 ± 0.66 g/d (P < 0.01) for CONTROL, LOW DDGS, and HIGH DDGS, respectively. Duodenal BCP flow tended to decrease with DDGS inclusion, but estimates were not affected by marker type. In addition, DDGS did not affect duodenal PCP supply and provided small amounts of duodenal YCP. Overall, the value of DDGS RUP determined in this study will contribute to a better understanding of the effect of this coproduct in ruminant nutrition.