Five ruminally and duodenally fistulated Angus x Hereford cows were used in a 5 x 5 Latin square to monitor intake, ruminal fermentation responses, and site and extent of digestion associated with providing increasing amounts of supplemental degradable intake protein (DIP). Cows had ad libitum access to low-quality, tallgrass-prairie forage (1.9% CP, 77% NDF) that was fed twice daily. The supplemental DIP (sodium caseinate; 90% CP) was infused intraruminally at 0630 and 1830 immediately before feeding forage. Levels of DIP were 0, 180, 360, 540, and 720 g/d. Each period consisted of 14 d of adaptation and 6 d of sampling. Forage OM intake increased quadratically (P < .01) with increasing supplemental DIP reaching a peak at the 540 g/d level. True ruminal OM and NDF digestion increased with the addition of 180 g/d supplemental DIP, but exhibited only moderate and somewhat variable responses when greater amounts of supplemental DIP were infused (cubic, P < or = .03). Microbial N flow and efficiency increased linearly (P < .01) with increasing supplemental DIP. However, a quadratic effect (P < .01) was observed for total duodenal N flow, which was maximized at 540 g/d supplemental DIP. A linear (P = .02) treatment effect was observed for ruminal fluid dilution rate. Total ruminal VFA and ammonia concentrations increased (P < .01) in response to DIP supplementation. In conclusion, increasing supplemental DIP generally improved forage utilization; intake of digestible OM was maximized when it contained approximately 11% DIP.

Three independent experiments were conducted each using 16 ruminally fistulated beef steers fed bermudagrass (8.2% CP, 71% NDF; Exp. 1), bromegrass (5.9% CP, 65% NDF; Exp. 2), or forage sorghum (4.3% CP, 60% NDF; Exp. 3) hays to evaluate the effects of increasing level of supplemental degradable intake protein (DIP) on forage utilization. In each experiment, steers were blocked by weight and assigned to one of four treatments, and hay was offered to each steer at 130% of average voluntary intake for the preceding 5-d period. Supplemental DIP (sodium caseinate) was placed in the rumen at 0700, immediately before feeding forage. Levels of DIP supplementation were .041, .082, and .124% BW; the control received no supplemental DIP. Following a 10-d adaptation, intake and total fecal output were measured for 7 d. In Exp. 1, neither forage OM intake (FOMI) nor fiber (NDF) digestion were influenced (P > or = .20) by increasing level of DIP supplementation. The DIP supplied by the bermudagrass hay was estimated to be 8.2% of the total digestible OM intake (TDOMI) for control steers. In Exp. 2, increasing level of supplemental DIP did not affect (P > or = .26) FOMI but tended to increase total OM intake linearly (TOMI; P = .10). The tendency for a rise in TOMI coupled with a slight numeric increase in digestion resulted in an increase (linear; P = .06) in TDOMI. In the treatment group in which the maximum TDOMI was observed (supplemental DIP treatment of .082% BW), total DIP intake constituted approximately 9.8% of the TDOMI. In Exp. 3, FOMI, TOMI, organic matter digestion (OMD), and TDOMI were improved (P < .01) by increasing amounts of supplemental DIP. Although there was some evidence of a tendency for a decrease in the magnitude of change in TDOMI in response to increasing DIP supplementation, a clear plateau was not achieved with the levels of supplement provided. When the highest level of supplemental DIP was fed, DIP constituted approximately 12.8% of the TDOMI. In conclusion, significant variation was observed among forage in the amount of DIP needed to maximize intake and digestion when expressed in relationship to the digestible OM.

The overall objective of these two studies was to evaluate the efficacy of using the proteolytic enzyme from Streptomyces griseus to estimate concentrations of ruminally degradable protein (RDP) in a wide array of forages. In the first study, alfalfa and prairie hays that previously had been evaluated in vivo for RDP were incubated in a replicated 3 x 3 factorial combination of enzyme concentrations (6.6, 0.66, and 0.066 activity units/ml of incubation medium) and incubation times (2, 4, and 48 h). Two treatment combinations (6.6 activity units for 4 h and 0.066 activity units for 48 h) yielded respective RDP estimates for alfalfa and prairie hay that were close to the known in vivo values. In the second study, 20 diverse forages were evaluated for RDP by using the in situ technique. These forages also were evaluated for RDP with the two enzyme concentrations identified in the first study, but incubation times were expanded to include 1, 2, 3, 4, and 5 h at the high concentration and 24, 30, 36, 42, 48, and 54 h at the low concentration. At the high enzyme concentration, r2 statistics from linear regressions of enzymatic estimates of RDP on corresponding estimates obtained by the in situ procedure were high (r2 > or = 0.898) at all incubation times; in addition, slopes (range = 0.88 to 1.00) and intercepts (range = -9.4 to 3.5%) approached unity and 0, respectively. At the lower enzyme concentration, r2 statistics were still good (> 0.81), but slopes (0.59 to 0.67) and intercepts (18.5 to 21.9%) for all incubation times did not meet the respective goals of unity and 0.

Newborn Holstein calves (n = 132) were used to evaluate three protein sources (soybean meal and soybeans roasted to an exit temperature of 138 or 146 degrees C) that contained 30, 45, or 52% RUP, respectively. Each was used with either raw corn or corn roasted to an exit temperature of 135 degrees C. Roasting of corn increased starch gelatinization from 34.8 to 118.6 mg of maltose equivalents/g of sample. Six pelleted isonitrogenous calf starters (18% CP) were fed for ad libitum consumption from 0.5 to 8 wk of age. Performance of calves fed soybeans roasted at 146 degrees C plus raw corn was superior to that of calves fed soybean meal plus raw corn but was similar to the performance of calves fed soybean meal plus roasted corn. Roasted corn tended to improve calf performance when used with soybean meal and, to a lesser degree, when used with soybeans roasted at 138 degrees C but depressed performance when used with soybeans roasted at 146 degrees C. Ruminal concentrations of NH3, butyrate, and total VFA and plasma concentrations of urea were higher for calves fed soybean meal. Ruminal pH and lactate, plasma NEFA, and blood BHBA concentrations were unaffected by starter. Performance was superior when calves consumed protein and corn sources with similar degrees of ruminal availability.

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