Effects of supplemental degradable intake protein on intake and Effects of supplemental degradable intake protein on intake and digestibility of forage sorghum hay digestibility of forage sorghum hay

Sixteen ruminall y fistulated beef steers with ad libitum access to forage sorghum hay were used to evaluate the effect of increasing level of degradabl e intake protein (DIP) on forage intake and digestion. Forage OM intake and total OM intake were enhanced with increasing level of DIP supplementation. Similarly, increases in total OM digestibil ity and total digest-ible OM intake (TDOMI) were evident. Compared with the negative control, TDOMI was approximatel y doubled at the highest level of DIP supplementation.


Introduction
Over the last decade, the approach to protein nutrition in ruminants has shifted from a crude protein (CP) system desc r ibed in the 1984 NRC to a metabolizable protein (MP) system describe d in the 1996 NRC.Metabolizable protein is defined as the true protein absorbed by the small intestine, which is supplied by microorganism s passing out of the rumen and by undegradable intake protein (UIP, i.e., escape protei n).The MP system accounts for the degradation of protein in the rumen and separates protein requirements into the needs of ruminal microorgani sms and the animal.Crude protein includes some that is ruminally degraded (degradable intake protein = DIP) and some that is not (UIP).
Beef cattle in the midwestern and plains states commonl y are fed forage sorghum hay as a roughage source .Frequently, this forage is of relatively low quality.Previous research on low-quality, tallgrass-prairie forage has demonstrated that DIP supplementation dramatically improved forage intake and utilization.In addition , the amount of DIP needed to maximize total digestible forage intake has been defined for this forage.However, information pertaining to the effec t s of DIP supplementation on forage sorghum hay is limited.This study was conducted to determine the impact of DIP supplementation on forage sorghum intake and digestion and to determine the amount of DIP needed to maximize intake of digestible material for this forage.

Experimental Procedures
Sixteen ruminall y fistulated beef steers (avg BW= 639 lb) were blocked by weight and assigned to one of four treatments to evaluate the effect of increasing level of DIP on forage intake and digestion.Each steer was offered forage sorghum hay at 130% of average voluntary intake for the preceding 5-day period.Supplementa l DIP (sodium caseinate; 91.6% CP, 100% DIP) was ruminally i nfused at 7 a.m., immediately prio r to feeding forage, at levels of .041,.082,and .1 23% BW/day (.045, .090,and .134%BW of casein as DM/day) The control treatment had no supplementa l DIP.The forage contained 55.8% NDF, 4.3% C P, and 51% DIP.Forage DIP (% of CP) was estimated using a single-point enzyme assay.
Following a 10-day adaptation, feed offered, feed refused, and total fecal output were measured for 7 days and used t o calculate digestibility coefficients.

Results and Discussion
Forage OM and total OM intakes increased (P<.01 ) in direct proportion to increasing level of DIP supplementation, although they tended to plateau (P=.13 and P=.15, r e spectively) at the higher levels.Similarly, a linear increase (P<.01) with a tren d toward diminishing returns (P=.17 and P=.14, r espectively) was evident for both total OM digestion (TOMD) and TDOMI.Total NDF intake and total NDF digestibility responded similarly.

a
Standard error of the mean (n = 16).b DM = dry matter.c OM = organic matter.d DOMI = digestible organic matter intake.e OMD = organic matter digestion.f NDFD = neutral detergent fiber digestion. g