Sources of variation of the in situ nylon bag technique.

Samples of a totally mixed ration and of soybean hulls were incubated in nylon bags in the rumen of four cows. Incubation times were 3, 6, 12, 24, 48 and 280 h. The 3- to 48-h incubations and determination of solubility were repeated in seven periods, whereas 280-h incubation was repeated in three periods. From these incubations, rates of disappearance of dry matter (DM), nitrogen (N) and neutral detergent fibre (NDF) were calculated. Statistical analyses for the following variances were calculated: between cows (Vc), between periods within cows (Vp) and between bags within periods within cows (Vb) only for DM, disappearance at incubation times of 24, 48 and 280 h. In the soluble fractions, Vp was generally higher than Vb, which might be explained by the bag treatment not being sufficiently standardized before and after washing. For rumen incubations, Vc was smaller than Vp or Vp + b. In most cases, variances were smaller at longer incubation times, except for Vc at 24- and 48-h incubations. The obtained data give a basis for selecting the numbers of bags, periods and cows in nylon bag experiments.