The number of bags to be incubated per sample, and the number of fistulated cows needed to estimate in situ ruminal degradation of Kinggrass (Pennisetum purpureum) dry matter (DMD %) were determined. Three rumen-fistulated cows and 10 bags per incubation time (3, 6, 9, 12, 24, 48 and 72 h) were used. Five grams of dried leaves were weighed per bag. Variance components for cows (Vc) and bags (Vb) per incubation time were estimated and used to calculate the standard error (SE) of mean DMD. The

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... quate number of bags to be used was the one that produced a value of 1 % (SE of the study). Precision increased as the number of cows increased, with mean SE of 1.66, 1.18 and 0.96 % for one, two and three cows, respectively. Precision remained more or less constant for incubation times of 3, 6, 9, 12 and 48 h, with respective values of 0.93, 0.81, 1.05, 0.92 and 1.08 %. The SE values doubled at 24 (2.15) and 72 (1.92) h. The EE diminished as the number of bags increased, being the values higher for one cow than for two or three cows, but these differences were slight when 10 bags were used. The use of two cows seems reasonable, since the increase in precision from one to two cows was greater than from two to three cows. Three bags were the minimum necessary to estimate the SE value of 1%.