Effect of changing the proportion, wheat: sorghum in dairy cow rations on carbohydrate digestibility and NAN flow to the intestine

Five rumen- and abomasum-cannulated cows in mid-lactation were fed ad libitum, for two periods of 21 d each, two isonitrogenous (17% crude protein) total mixed rations (TMR) composed of 45% sorghum-wheat combination, 38% corn silage, 12% soybean meal, urea, buffers and minerals. The two TMRs differed in their sorghum (S): wheat (W) ratio, which was 7030 in S and 3070 in W treatments. Dry matter intake was 17.2 and 16.6 kg d−1 in S and W, respectively. Effect of treatments on the digestibility of carbohydrate constituents and N metabolism was examined. l detergent fiber (NDF)-polysaccharides digestibility in stomachs and the entire gastrointestinal tract (GIT) was significantly higher in S than in W. The digestibility of neutral detergent soluble (NDS)-α-glucans was significantly higher in W than in S. More of NDS-α-glucans escaping ruminal fermentation was digested in the intestine of the S cows as compared with the W cows. The digestibility in entire GIT of total NDS-monosaccharide residues was 93% in W and 86% in S. Digestibility values of total carbohydrates in stomachs (49% and 52%) and entire GIT (79% and 80%) of S and W cows were similar. W cows were similar in rumen pH (6.15 and 6.13), rumen volatile fatty acids concentrations (133 and 136 mM) and rumen NH3 concentration (21 and 23 mg per 100 ml). Non-ammonia-N flow through the abomasum (410 and 403 g d−1) and N absorption in the intestine (67%) were also similar in S and W.