Supplementation based on protein or energy ingredients to beef cattle consuming low-quality cool-season forages: I. Forage disappearance parameters in rumen-fistulated steers and physiological responses in pregnant heifers.

摘要

Two experiments evaluated the influence of supplement composition on ruminal forage disappearance, performance, and physiological responses of Angus x Hereford cattle consuming a low-quality cool-season forage (8.7% CP and 57% TDN). In Exp. 1, 6 rumen-fistulated steers housed in individual pens were assigned to an incomplete 3x2 Latin square design containing 2 periods of 11 d each and the following treatments: (1) supplementation with soybean meal (PROT), (2) supplementation with a mixture of cracked corn, soybean meal, and urea (68:22:10 ratio, DM basis; ENER), or (3) no supplementation (CON). Steers were offered meadow foxtail (Alopecurus pratensis L.) hay for ad libitum consumption. Treatments were provided daily at 0.50 and 0.54% of shrunk BW/steer for PROT and ENER, respectively, to ensure that PROT and ENER intakes were isocaloric and isonitrogenous. No treatment effects were detected on rumen disappearance parameters of forage DM (P>=0.33) and NDF (P>=0.66). In Exp. 2, 35 pregnant heifers were ranked by initial BW on d -7 of the study, allocated into 12 feedlot pens (4 pens/treatment), and assigned to the same treatments and forage intake regimen as in Exp. 1 for 19 d. Treatments were fed once daily at 1.77 and 1.92 kg of DM/heifer for PROT and ENER, respectively, to achieve the same treatment intake as percent of initial BW used in Exp. 1 (0.50 and 0.54% for PROT and ENER, respectively). No treatment effects (P=0.17) were detected on forage DMI. Total DMI was greater (P<0.01) for PROT and ENER compared with CON and similar between PROT and ENER (P=0.36). Accordingly, ADG was greater (P=0.01) for PROT compared with CON, tended to be greater for ENER compared with CON (P=0.08), and was similar between ENER and PROT (P=0.28). Heifers receiving PROT and ENER had greater mean concentrations of plasma glucose (P=0.03), insulin (P<=0.09), IGF-I (P<=0.04), and progesterone (P=0.01) compared to CON, whereas ENER and PROT had similar concentrations of these variables (P>=0.15). A treatment x hour interaction was detected (P<0.01) for plasma urea N (PUN), given that PUN concentrations increased after supplementation for ENER and PROT (time effect, P<0.01) but did not change for CON (time effect, P=0.62). In conclusion, beef cattle consuming low-quality cool-season forages had similar ruminal forage disappearance and intake, performance, and physiological status if offered supplements based on soybean meal or corn at 0.5% of BW.