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Estimating Particle Turnover in the Rumen of Meal-Fed Beef Steers: Procedural Evaluations¹

U. S. Department of Agriculture^,6, Miles City, MT 59301 and New Mexico State University Las Cruces 88003

Abstract

Twelve ruminal-cannulated beef steers (average wt 453 kg) were used in two experiments to evaluate effects of sampling site, mathematical model, method of pulse-dose administration of marker and intraruminal mixing on particle turnover estimates. In both experiments, steers were offered prairie hay once daily in amounts sufficient to allow ad libitum consumption (i.e., meal fed). Actual dry matter (DM) consumption averaged 19.5 g DM/kg body weight across experiments. Each 16-d experiment included a 10-d adaptation and 6-d trial period. At the beginning of each experiment, steers were randomly assigned to treatments. In Exp. 1, treatments were 1) rectal sampling, 2) ruminal sampling-ruminal contents mixed before subsampling and 3) ruminal sampling-ruminal contents not mixed before subsampling. In Exp. 2, serial fecal samples were collected following intraruminal administration of a pulse dose of Yb-labeled forage by 1) placement of loose forage on top of hay mat, 2) placement of loose forage packed in gelatin capsules on top of hay mat and 3) stratification of loose forage from middorsal to midventral region of the rumen. Particle turnover was estimated from serial digesta or fecal samples following administration of a pulse dose Yb-labeled prairie hay. Fecal Yb excretion curves from both experiments were fitted to one- and two-compartment, time-dependent (1CMPT-TD, 2CMPT-TD) models and a two-compartment, time-independent (2CMPT-TI) model. Within fecal models, comparisons were made among slopes associated with the slower turnover process. Ruminal Yb concentrations from Exp. 1 were transformed to natural logarithms and regressed on time. In both experiments, minor differences were evident among models used to fit fecal Yb excretion curves; however, differences among models did not influence interpretation of treatment effects. In Exp. 1, turnover rates from ruminally sampled steers were less (P<.05) than rates from rectally sampled steers. Intraruminal mixing did not influence (P>.10) turnover rates. In Exp. 2, no difference (P>. 10) was noted among methods of pulse-dose administration of marker. Under meal-feeding conditions, sampling site exhibited the strongest potential for modifying estimated turnover rate constants.

¹ Publication has been approved by the Director of the Montana Agr. Exp. Sta., Journal Ser. No. J-1782.

² Present address: Dept. of Anim. Sci. and Ind., Call Hall, Kansas State Univ., Manhattan 66506.

³ Address reprint requests to this author.

⁴ Fort Keogh Livestock and Range Res. Lab., USDA-ARS, Route 1, Box 2021, Miles City, MT 59301.

⁵ Dept. of Anim. and Range Sci., Box 3-I, New Mexico State Univ., Las Cruces 88003.

⁶ Agr. Res. Serv., Fort Keogh Livestock and Range Res. Lab. with Montana Agr. Exp. Sta. cooperating.