Correcting the calculation of extent of degradation to account for particulate matter loss at zero time when applying the polyester bag method

The established calculation of ruminal extent of degradation using the poly ester bag method overestimates extent. The wash fraction, at least in part, is subject to losses from the rumen due to passage. Four formulae are prop osed to minimize this risk of overestimation. Four options are considered: 1) passage losses for particulate matter escaping from the bag at zero time are according to the particulate fractional passage rate, 2) the liquid ra te, and 3) the average of the two, and additionally 4) there is no instantl y degradable fraction. The established and proposed formulae were examined using polyester bag data for six forages: grass silage, fresh ryegrass, fre sh white clover, alfalfa hay, mixed grasses hay, and hay from a permanent m ountain meadow. The established formula gave appreciably higher estimates o f extent of degradation in all cases. Overestimation was at least 6.9, 4.9, 2.9, 2.1, 2.4, and 4.5%, respectively, for the six forages.