Passage of stable isotope-labeled grass silage fiber and fiber-bound protein through the gastrointestinal tract of dairy cows

Abstract

Fractional passage rates are required to predict nutrient absorption in ruminants but data on nutrient-specific passage kinetics are largely lacking. With the use of the stable isotope ratio (δ) as an internal marker, we assessed passage kinetics of fiber and fiber-bound nitrogen (N) of intrinsically labeled grass silage from fecal and omasal excretion patterns of δ13C and δ15N. In a 6×6 Latin square, lactating dairy cows received grass silages [455g/kg of total diet dry matter (DM) ] in a 2×3 factorial arrangement from ryegrass swards fertilized at low (45kg of N/ha) or high (90kg of N/ha) levels of N and harvested at 3 maturity stages. Feed intake (16.7±0.48kg of DM/d; mean ± standard error of the mean) and milk yield (26.7±0.92kg/d) increased at the high level of N fertilization and at decreasing maturity. Nutrient digestibility decreased with increasing plant maturity, particularly at the high level of N fertilization, essentially reflecting dietary treatment effects on the nutritional composition of the grass silage. Fractional rumen passage rates (K1) were highest and total mean retention time in the gastrointestinal tract (TMRT) was lowest when based on the external marker chromium mordanted fiber (Cr-NDF; 0.047/h and 38.0h, respectively). Fecal δ13C in the acid detergent fiber fraction (13CADF) provided the lowest K1 (0.023/h) and the highest TMRT (61.1h) and highest peak concentration time (PCT; 24.3h) among markers. In comparison, fecal fiber-bound N (15NADF) had a considerably higher K1 (0.032/h) and lower TMRT (46.4h) than 13CADF. Total N (measured with 15NDM) had a comparable K1 (0.034/h) to that of 15NADF but provided the highest fractional passage rates from the proximal colon-cecum (K2; 0.37/h) and lowest PCT (17.4h) among markers. A literature review indicated unclear effects of grass silage maturity on K1 and unknown effects of N fertilization on K1. Our study indicated no effect of advancing maturity on fecal K1 and a trend for K1 to increase with the high level of N fertilization. Parameter K2 increased, whereas PCT and TMRT generally decreased with the high level of N fertilization. Omasal digesta sampling largely confirmed results based on fecal sampling. Results indicate that the use of δ13C and δ15N can describe fiber-specific passage kinetics of forage.