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Forage Characteristics Related to Intake, Digestibility and Gain by Ruminants^1,2,

Texas Agricultural Experiment Station, Angleton 77515

Abstract

Seven sorghum x sudan hays [Sorghum bicolor (L.) Moensch] and five Bermudagrass hays [Cynodon dactylon (L.) Pers.] , harvested at three locations and at various stages of maturity, were fed to yearling steers to determine intake and gain, and to growing wethers to determine digestibility. Animal responses and laboratory analyses all showed a decline in forage value with increasing maturity within forage species and location. Except when forage protein was <6%, body weight gain appeared to be solely a function of digestible energy intake (r = .98). Intake was a better indicator of weight gain (r = .93) than was digestibility (r = .77), with no significant relationship (r = .49) between intake and digestibility. Fecal excretion was more closely related to intake (r = .79), and using it as a covariate to digestibility yielded the conceptually strong association with intake (r = .97). Hemicellulose to cellulose ratio had no relationship to digestibility but was highly correlated with neutral detergent fiber (NDF) intake (r = .94), possibly through common associations with NDF excretion rate. Dry matter and digestible organic matter intakes were closely correlated to acid detergent fiber (ADF) (r²>.86) and protein content (r²>.73). Lignin content based on dry matter, NDF or ADF was generally not a good indicator of fiber digestibility. The relationship was significant (r = –.91) only for lignin to ADF ratio in the sorghum x sudan hays. Correlation coefficients for the relationships between lignin and hemicellulose digestibility and lignin and digestible organic matter were –.83 and –.84, respectively. In vitro dry matter disappearance (IVDMD) predictor of digestible organic matter (r = .91), but the low regression coefficient (.49) suggests that unadjusted IVDMD values should be used for ranking purposes only.

¹ TA 15247, Texas Agr. Exp. Sta.

² The author gratefully acknowledges the advice of M. E. Riewe in the preparation of this manuscript.

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