(Key Words: Stockers, Supplementation, Bermudagrass.)

Declining forage quality during late summer months often results in reduced performance of grazing cattle. Previous research has shown that high protein supplement increases intake and digestibility of prairie hay harvested during mid summer (Guthrie et al., 1984). Furthermore, daily gains of stocker cattle grazing bermudagrass pastures during late summer was increased by more than 30% when supplemented with 1 lb per day of soybean meal (Cantrell et al., 1985). Feeding greater amounts of energy in combination with supplemental protein should increase weight gain during the later summer grazing period. However, research suggests that energy supplements based on feedstuffs high in starch, such as grain, reduces forage digestibility (Chase and Hibberd, 1985) compared with energy supplements formulated with highly digestible fiber sources, such as corn gluten feed or soybean hulls (Hibberd et al., 1986). This study was conducted to determine the feasibility of supplementing additional energy, and to compare supplemental energy sources for stocker cattle grazing bermudagrass pasture during late summer.

Forty-five fall born Angus and Angus x Hereford heifers (initial weight = 565 ± 8 lb) were used in a completely random design to study the effects of energy source and protein supplementation on summer weight gain. Heifers grazed a 22-acre bermudagrass pasture from July 1 through September 30 (91 days). The cattle were weighed and treated with EprinexÔ on June 24 and randomly assigned to one of five supplementation treatments (Table 1). Treatments consisted of: 1) no supplement (Control), 2) 3 oz per head per day of a mineral mixture with 50 mg monensin per ounce (Mineral), 3) 5 lb per head per day digestible fiber energy supplement (15% CP) with 30 mg monensin per lb (HF), 4) 5 lb per head per day grain based energy supplement (15% CP) with 30 mg monensin per lb (HG), and 5) 2.5 lb per head per day digestible fiber supplement (30% CP) with 60 mg monensin per lb (LF). On July 1, heifers were re-weighed, tagged according to treatment assignment and supplementation was initiated. Interim and final weights were recorded on d 49 and d 91, respectively. All weights were recorded after overnight (15 h) removal from feed and water. The pasture was fertilized with 100 lb N per acre and sprayed with herbicide on May 15. Forage availability was excellent to adequate throughout the experiment with estimated forage dry matter of 3,482 lb per acre on July 1 and 2,683 lb per acre on September 30. Supplements were prorated for 6 d per week feeding. Each heifer was individually supplemented in a feeding barn located adjacent to the bermudagrass pasture. Data were analyzed for a completely random design. Preplanned comparisons among treatments were made using orthogonal contrasts.

Weight Gain. Weight gain did not differ throughout the experiment between Control and Mineral fed heifers (Table 2). This is likely due to the insufficient and inconsistent consumption noted for the Mineral treatment, resulting in inadequate monensin intake. Daily gain was significantly less (P<.01) for non-supplemented cattle (Control and Mineral) compared with cattle receiving a supplement (1.18 vs 1.59 during Period 1 and .82 vs 1.55 during Period 2). Rate of weight gain did not differ between the two energy sources (HF and HG, P>.1). Overall weight gain for cattle fed HF and HG supplements was greater (P=.1) compared with LF fed heifers.

Supplement Conversion. Pounds of feed required to achieve one additional pound of weight gain beyond non-supplemented cattle was 8.27, 8.13 and 5.43 for HF, HG and LF, respectively.

Actual vs Predicted Response to Supplement. The 1996 beef cattle NRC was used to compare actual and predicted response from each supplement (Table 3). The following procedure was used:

Predicted response from supplement was very similar to actual response. Minor adjustments in forage digestibility were needed to match predicted weight gain with actual weight gain of cattle receiving supplements. This would suggest that supplements had minimal influence on forage digestibility, assuming forage intake predictions are reasonable. The model predicted a 31% decrease in forage intake when cattle were supplemented with .7% of body weight HF and HG. When cattle were fed .35% LF, the model predicted a 15% reduction in forage intake. Average estimated replacement rate of supplement for forage was .96. Logically, actual forage intake, digestibility and metabolizable protein supply would need to be measured in order to determine the accuracy of these predictions and/or assumptions.

Cantrell, J. et al. 1985. Okla. Agr. Exp. Sta. Res. Rep. MP-117:254.

Chase, C. C. and C. A. Hibberd. 1985. Okla. Agr. Exp. Sta. Res. Rep. MP-117:207.

Guthrie, M. J. et al. 1984. Okla. Agr. Exp. Sta. Res. Rep. MP-116:156.

Hibberd, C. A. et al. 1986. Okla. Agr. Exp. Sta. Res. Rep. MP-118:192.

Appreciation is expressed to Farmland Industries and Elanco Animal Health for financial support of this experiment.