Effects of various supplemental starch and protein levels on Effects of various supplemental starch and protein levels on ruminal fermentation and liquid passage of beef steers fed ruminal fermentation and liquid passage of beef steers fed tallgrass-prairie hay tallgrass-prairie hay

The effect of supplements containing various proportions of degradable intake protein (DIP) and starch on ruminal dig estion characteristics of forage-fed beef steers was evaluated. Fluid passage rates, ruminal ammonia (N H), 3 and total volatile fatty acid (VFA) concentrations increased as the amount of supplemental DIP increased. Starch infused at .3% of BW increased molar proportion s of propionate and butyrate and decreased acetate, compared to feeding DIP alone. However, proportions of branched-chai n VFA increased with DIP at all levels of starch infusion. Total digestible organic matter intake (TDOMI) was increased with each addition of DIP; however, infusing starch within a DIP level decreased TDOMI. Providing supplemental DIP is more important for improvin g the use of low-quality, tallgrass-prairie hay than is ruminally available starch.


Introduction
Intake and digestion of low-protein forages by beef cattle are known to increase when supplemental degradab l e intake protein (DIP) is fed.Precise feeding recommendations for DIP need to be established, because protein-based feedstuff s are expensive.Furthermore, it is unclear how other supp l ement components, like starch, affect animal response to DIP supplementation.Studies investigating the effect of supplement s containing various proportions of DIP and starch on ruminal digestion characteristics are needed to defin e desirable supple-ment co mpositions.This study was designed to examine the interactive effects of supplemental DIP and starch on ruminal fermentation and liquid passage rate of steers consuming lowquality hay.

Experimental Procedures
Thirtee n beef steers (average initial body weigh t = 570 lb) were used in a 13-treatment, four-period , Latin square.Treatments were arrang ed as a 3 x 4 factorial plus an unsupplemented control an d consisted of four DIP levels (ca sein infused at .03, .06,.09,and .12% of BW) superimposed on three starch levels (none or corn starch grits i nfused at .15 and .3% of BW).All steers had ad libitum access to tallgrass-prairi e hay (5% CP).Forage refusals were measured, and new forage was offered once daily.Supplements were infused intraruminall y immediately before forage was fed.Following an adequate adaptation period, digestibilit y was determined via total fecal collection .Subsequently, ruminal VFA, N H, 3 and fluid passage rates were evaluated by collecting multiple samples of ruminal fluid throughout a given day.

Results and Discussion
Productio n responses by beef cattle are driven, to a la rge degree, by the total amount of digestibl e organic matter (digestible forage organic matter + digestible supplement organic matter; TDOMI) th at is consumed.Supplementation programs that increase TDOMI can be said to augment total energy supply to the animal.Steers receiving no supplement in our study had lower TDOMI than supplemented steers (Table 1).In general, TDOMI was increased when DIP was provided but was decreased when starch was added within a given level of DIP supplementation.We interpret this result to mean that DIP is more critical to achievin g optimal use of low-protein forages than is ruminally degradable starch.
Effects of starch and DIP supplements on ruminal fermentation and rate of fluid passage were consisten t with changes in TDOMI.Average levels of ruminal NH were low for all 3 treatments (range = trace to 1.5 mM), likely becaus e of the low protein level in the basal forage and efficient use by ruminal microbes.Rumina l NH increased as supplemental DIP 3 increased , regardless of starch level.Steers receiving only DIP had greater ruminal NH 3 than steers receiving starch at .15 or .3% of BW, probably because of the greater capability of starch-di gesting bacteria to compete for NH 3 compared with fiber-digesting bacteria.
Total volatile fatty acid (VFA) concentrations are correlated with ruminal diet digestibility.Unsupplemented steers had lower total VFA than supplemented steers.Total VFA concentration increa sed with supplemental DIP but was not affected by starch infusion.
Proportion s of individual ruminal VFA are useful indicators of the type of fermentation predominating.Supplemented st e ers had greater proportion s of isobutyrate, valerate, and isovalerat e than unsupplemented steers, which likely was due to the provision of precursors for these VFA in the supplemental DIP.Steers receiving only DIP had h i gher acetate and lower propionate and butyrate tha n steers fed DIP plus starch at .3%BW.Greater proportions of acetate in steers give n only DIP indicated increased fermentatio n of structural carbohydrates from the forage.Conversely, increased propionate and butyrate in steers fed the highest level of starch suggested that ruminal microbes were less reliant on forage fiber as an energy source.
Frequently , increased rate of nutrient passage from the rumen is associated with higher feed intake.Rumina l liquid passage rate in our study became more rapid as the amount of supplemental DIP i ncreased but was not altered by starch supplementation.This stud y supports the contention that DIP is the primary factor limi t ing the effective use of low-quality , tallgrass-prairie forage by beef cattle.In general, total VFA concentration, rate of ruminal liquid passage, and TDOMI were greates t when DI P fell between 10 and 12.5 % of TDOMI.Infusion of ruminally degradable starch altered patte rns in VFA and NH concen- supplement vs. s upplement, 2 = DIP only, 3 = DIP + .15%BW starch, 4 = DIP + .3%BW starch, 5 = DIP 3 only vs. DIP + .15%BW starch, 6 = DIP only vs. DIP + .3%BW starch