Evaluation of Supplementation Programs for Growing Cattle Grazing Tall Fescue1

Eighty steers and heifers were used to determine the effects of supplemental energy or undegradable intake protein (UIP) on the performance of growing cattle grazing tall fescue. Treatments included 1) no supplemental feed offered (control), 2) 0.91 kg/d of supplemental corn supplying 26 g UIP/d, 3) low-escape protein-0.91 kg/d of a 22% CP supplement supplying 117g UIP/d, or 4) high-escape protein-0.91 kg/d of a 42% CP supplement supplying 239 g UIP/d. Effective N degradabilities of the supplements were 63.6, 43.8, and 37.5%, respectively. All supplements also supplied 224 mg of lasalocid/ kg. Calf ADG was increased by an average of 0.11 kg by supplementation (control vs supplement, P = 0.02), and supplement type did not affect ADG (linear and quadratic responses of UIP, P > 0.36). The ratio of in-situ ruminallydegraded OM to ruminally-degraded CP (RDOM:RDCP) can be an effective predictor of the balance of ruminally available N and energy for designing supplementation programs for grazing cattle. Thus, the ratio of forage RDOM:RDCP was determined at 0, 8, 16, 24, 36, and 48 h of in situ incubation. The ratio of RDOM:RDCP of the forage samples remained essentially unchanged after 24 h of incubation, indicating that a single point in situ analysis can be used to determine this ratio. In conclusion, calves grazing tall fescue with 24-h in situ RDOM:RDCP ratios of 4.32 or less benefited equally from grain-based energy supplements or supplements supplying 117 and 239 g of UIP daily.