Comparative Digestion of Dry Annual Range Forage by Cattle and Sheep

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Abstract
Apparent digestion coefficients were calculated by ratio techniques using composition of esophageal fistula forage samples and total fecal collections from ruminal fistulated animals. All animals were grazed together and handled similarly on an annual range where herbage decreased from 1490 to 420 lb./acre during the summer. The internal indicators were lignin on a dry matter, silica-free, and organic matter basis and silica on a dry matter basis. The ruminal fistulated animals were also used in conventional digestibility trials on drylot. Digestion coefficients calculated by silica ratio varied widely during the summer. There was no significant difference in digestion coefficients calculated by lignin ratio when forage and fecal composition were on dry matter, or silica-free, or organic matter basis. Sheep were equal to or lower than cattle in early summer but higher in late summer for lignin ratio digestion coefficients for dry matter, ether extract, and other carbohydrates. Sheep always digested more crude protein and less cellulose than did cattle. Digestion coefficients for sheep and cattle were closer during early than late summer for most constituents and never differed significantly for gross energy. Total fecal collection and nylon bag and artificial rumen microdigestion estimates were used to calculate forage intake and, subsequently, macrodigestion coefficients. Lignin was 4% digestible by this method; therefore, most digestion coefficients determined from microdigestion data were higher than those determined by lignin ratio. All procedures except silica ratio gave comparable seasonal trends in digestibility and differences between sheep and cattle. Variations among the esophageal fistulated animals (forage animals) caused greater variations in digestion coefficients than did variations among ruminal fistulated animals (fecal animals). Averaged over all constituents, from 10 to 13 forage or fecal animals of either class would be required to estimate digestion coefficients within 10% of the mean with 90% confidence. The average numbers of forage plus fecal animals of both classes required to estimate digestion coefficients were 29, 25, 6, 5, and 3 for crude protein, ether extract, other carbohydrates, cellulose, and dry matter, respectively. Variability in estimates of digestibility of gross energy was similar to that for digestibility of cellulose. For optimum use of a given number of animals during the summer, 1.2:1 forage to fecal animals or sheep to cattle should be used. Cattle and sheep did not differ in ability to digest pelleted alfalfa on drylot, but there were within-class differences. Four wethers and two steers were sufficient on drylot to estimate most digestion coefficients within 10% of the mean with 95% confidence. Averaged over all trials, about six wethers and four steers would be required to estimate cellulose digestibility. Variability on the range was 50% more than in drylot. The ruminal fistulated animals maintained their weight during 3 months grazing on dry annual range when digestible energy crude protein consumption was lower than level: calculated or reported in the literature for maintenance of animals on drylot. Copyright © 1964. American Society of Animal Science . Copyright 1964 by American Society of Animal Science

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