Date of Award


Document Type


Degree Name

Doctor of Philosophy (PhD)

First Advisor

W. M. Craig


The effect of diet and time after feeding on microbial crude protein (CP) and dry matter (DM) associated with undigested rumen particles was investigated using diaminopimelic acid (DAPA) as a microbial marker. In a subsequent experiment, in situ CP and DM digestibilities were corrected for microbial contamination of non-masticated and masticated (M) forages. Particle-associated (PA) microorganisms had a higher (P $<$.01) concentration of organic matter (OM) than fluid microorganisms 2 and 23 h after feeding. Bacterial fraction had a higher (P $<$.01) concentration of CP than protozoal fraction. However, the quantity of OM, CP and DAPA per ml of strained rumen fluid equivalent were higher (P $<$.01) in the protozoal than in respective bacterial fractions. In experiment two, CP and DAPA concentrations (%) and quantity (mg/ml SRFE) of OM, CP and DAPA were constant over time (P $>$.10) in fluid, loosely-associated (LA) and firmly-associated (FA) populations. Mean CP and DAPA concentrations were in the order fluid $>$ LA $>$ FA and fluid = FA $$ fluid (P =.07). Over 50% of the PA population remained after extensive washing of particles. Microbial DM and CP contamination, using values obtained from the FA population, ranged from 10.3 to 22.3% and 46.3 to 95.3% of residual DM and CP respectively. All contamination increased (P $<$.01) with time of incubation, except DM contamination of alfalfa hay (AH) and MAH. Corrected digestibilities of DM (DMD) and CP (CPD) were higher (P $<$.01) than apparent. Lower digestion lag times and faster rates of digestion were observed in corrected compared with apparent DMD and CPD. No differences (P $>$.10) in lag times for digestibility of neutral (NDFD) and acid detergent fiber (ADFD) occurred among forages. Rate of NDFD was faster (P $<$.01) in AH than in other forages. The quantity of microbial CP contamination (mg/g DM) was significantly correlated with corrected DMD, CPD, NDFD and ADFD. Trends for masticated forages were similar to non-masticated, and except at some time points, showed no differences (P $>$.10) in digestibility, lag time or rates.