Effect of dietary concentration of total nonstructural carbohydrate on energy and nitrogen metabolism and milk production of dairy cows.

Two complete blended diets with a ratio of concentrate: silage dry matter of 60:40 were fed to 12 Holstein cows in the first 12 wk of lactation in an incomplete changeover arrangement of treatments. Diets differed (dry basis) in content of total nonstructural carbohydrate (24.9% versus 32.9%), neutral detergent fiber (37.0% versus 32.1%), and hemicellulose (19.6% versus 15.7%) but were similar in amounts of lignin, crude protein, soluble nitrogen, and acid detergent insoluble nitrogen. The diet with more total nonstructural carbohydrate was associated with greater dry matter intake as a percentage of body weight and greater yields of milk and solids-not-fat. Cellulose digestibility and mean rumen ammonia concentration were lower with this diet. Despite similar protein solubilities, the diet with more total nonstructural carbohydrate contained more rumen degradable nitrogen (80% versus 60%) but similar amounts of rumen degradable dry matter (82% versus 79%). The metabolizable energy of this diet was used more efficiently for the combined functions of maintenance and production, and net energy for lactation was larger (2.2 versus 1.9 Mcal/kg dry matter), as measured calorimetrically.

Effects of solids and liquid flows on fermentation in continuous cultures. II. Nitrogen partition and efficiency of microbial synthesis.

The effects of varying solids retention time (SRT) and liquid dilution rate (D) on N partition and microbial efficiency were studied in vitro with a dual flow continuous culture system. SRT's of 14.3, 22.0 and 29.7 hr and D's of .07, .11 and .15 volumes/hr were used. Ammonia N accounted for 2.69 to 7.30% of total effluent N. Highest values were observed with 29.7-hr SRT at .11 and .15/hr D's, and were associated with a slight decrease in microbial N. Between 1.17 and 2.50 g microbial N were produced per 24 hr, accounting for 31.34 to 50.49% of total effluent N. Daily output of microbial N increased (P less than .05) with increasing D at the lowest SRT but showed little or no change at the 22.0- and 29.7-hr SRT's. Feed bypass N was inversely related to microbial N and accounted for 44.91 to 65.10% of total effluent N. Microbial cell yields per mole ATP (YATP) ranged from 10.40 to 24.41 and tended to increase with decreased SRT more than with increased D, although responses were variable. Efficiency of microbial N synthesis ranged from 15.75 to 23.91 g microbial N/kg digested dry matter (DDM), with trends similar to those seen for YATP.