Abstract/Summary

The objective was to determine the effect of dietary ratio of neutral detergent fibre (aNDFom) to starch within diets differing in grass to maize silage ratio on rumen function, diet digestion, serum haptoglobin, and production of lactating dairy cows. Four isonitrogenous diets were formulated with a forage to concentrate ratio of 50:50, with the forage proportion containing either a high or low ratio of grass silage to maize silage (82:18 [GS] or 18:82 [MS] on a dry matter [DM] basis, respectively) and the concentrates containing either a high (F) or low (S) aNDFom to starch ratio, giving 4 dietary ratios of aNDFom to starch. Diets were fed to 4 early lactation Holstein dairy cows in a 4 × 4 Latin square design with 28-d periods. Feed intake, eating behaviour, milk production and composition, total tract digestion, nitrogen (N) excretion, aNDFom passage rate and in-situ degradation, rumen pH, and serum haptoglobin were measured during the last week of each period. Cows fed the MS diets consumed 1.34 kg/d more DM (P = 0.047) and 2.38 kg/d more starch (P = 0.001) compared to GS diets and produced 2.46 kg/d more milk (P = 0.038). Milk fat concentration was higher (+2.88 g/kg) for cows fed GS diets compared to MS diets (P = 0.007), while cows fed S concentrates had a higher milk fat concentration (+1.8 g/kg) irrespective of forage source (P = 0.033). Digestibility of aNDFom was higher (+0.106 kg/kg) for GS diets than for MS diets (P = 0.004). Similarly, aNDFom digestibility was higher (+0.057 kg/kg) for F concentrates (P = 0.031). Rumen and total-tract particle retention times were higher (+11.9 and +9.1 h, respectively) for cows fed GS diets (P = 0.009 and P = 0.037, respectively). Milk N yield/N intake was higher for the MS diets versus GS diets (P = 0.045), due to a greater (+130 g/d) milk protein yield (p = 0.015). Cows fed the MS diets spent 187 min/d more with rumen pH below 5.8 compared to GS diets (P = 0.006). Serum haptoglobin concentration, a purported marker of gut inflammation, was 5.3 ng/ml higher for cows fed S concentrates versus F concentrates (P = 0.023). In conclusion, changes in concentrate aNDFom:starch ratio had little effect on DM intake, milk yield and composition, rumen function, and eating behaviour compared to effects of silage source (MS vs GS), where replacing a portion of diet GS with MS increased feed intake, milk yield, rumen passage rate, and N digestion, but also reduced fibre digestion and milk fat concentration. These observations suggest a greater effect of forage type on lactation performance than concentrate type per se under the conditions of the current study.