COMPARISON OF EFFECTIVE DEGRADABILITY WITH DRY-MATTER DEGRADABILITY MEASURED AT MEAN RUMEN RETENTION TIME FOR SEVERAL FORAGES AND FORAGE - CONCENTRATE DIETS

Dry matter degradability (DMdg) was determined for each of alfalfa (AA), orchardgrass hay (OG), alfalfa silage (AS), maize silage (CS), 70:30 cracked maize and OG (7OG), and 70:30 barley straw and AA (7OS) from feed incubations in nylon bags in mature rumen-cannulated steers in a 6 x 6 Latin square experiment for time periods equal to the mean rumen retention times of the listed feeds. The DMdg values were compared with the effective degradabilities of the feeds, which were estimated from the respective readily soluble fractions, potentially digestible fractions, rates of digestion and rates of passage of the feeds. The DMdg of the feeds at rumen retention times obtained from the reciprocal of the exponential rates of Cr decline in either feces or rumen contents were 0.69, 0.69; 0.70, 0.68; 0.54, 0.53; 0.62, 0.64; 0.69, 0.71; and 0.48, 0.49% for AA, OG, CS, AS, 7OG and 7OS, respectively. Estimated effective degradabilities of the respective feeds were 0.64, 0.63; 0.63, 0.64; 0.49, 0.47; 0.61, 0.61, 0.61, 0.60; and 0.43, 0.43% when the rate of passage was estimated as the exponential rate of marker decline in either feces or rumen contents, respectively. DMdg at the rumen retention time of feeds correlated (r2 > 0.9, P < 0.0001) with the effective degradabilities of the feeds.