Factors affecting the intake of grass silage by cattle and prediction of silage intake

A partially balanced change-over design experiment involving 192 beef steers, which were initially 14 months old and 415 kg live weight, was carried out to determine the intakes of 136 silages from commercial farms in Northern Ireland. Each silage was offered ad libitum as the sole food to 10 animals, with eight silages offered in each of 17 periods over 2 years. A standard grass hay was offered to 16 animals in each period to enable period effects on intake to be removed. Detailed chemical and biological compositions of the silages were also determined. The ranges for pH and dry matter (DM), crude protein, ammonia-nitrogen and apparent digestible organic matter (in vivo) concentrations in the silages and silage dry DM intakes were 3.50 to 5.49 (s.d. 0.396); 155 to 413 (s.d. 4.31) g/kg; 79 to 212 (s.d. 24.4) g/kg DM; 45 to 384 (s.d. 63.2) g/kg total nitrogen; 528 to 769 (s.d. 58) g/kg DM and 4.3 to 10.9 (s.d. 1.13) kg/day respectively. Relationships between intake and individual parameters or groups of parameters have been developed using simple and multiple linear regression analysis and partial least-squares analyses. Silage intake was closely related to factors which influence the extent of digestion and rate of passage of the material through the animal, as indicated by the strong relationships (R-2 of regressions = 0.28 to 0.50) with in vivo apparent digestibility and rumen degradability and the concentrations of the fibre and nitrogen factors. Intake was poorly correlated with factors such as pH, total acidity, buffering capacity and the concentrations of lactic, acetic and butyric acids (R-2 of regressions = zero to 0.11). Near infrared reflectance spectrometry (NIRS) provided the best fit relationship with intake (R-2 of relationship = 0 90). The results also indicate that the intake potential of silages can be directly predicted with a high degree of accuracy from the NIRS of both dried and undried samples of silage, provided the appropriate sample preparation and scanning methods are used.