Effects of Starch, Fiber, or Protein Supplementation on Intake and Digestion of Low-Quality Tall Fescue Hay by Growing Dairy Heifers

Holstein heifers (n = 20; initial BW = 270 ± 46 kg) were used in 3 consecutive randomized, complete-block experiments to measure the effects of ruminally degradable protein (RDP), NDF, and ruminally degradable starch (RDS) supplementation on utilization of tall fescue hay (8.7% CP, 73.1% NDF, 46.5% ADF). Supplementation rates were 0, 0.05, 0.10, 0.15, or 0.20% of BW per day of RDP delivered via soybean meal; 0, 0.16, 0.33, 0.49, or 0.65% BW per day of NDF delivered via soybean hulls; and 0, 0.10, 0.20, 0.30, or 0.40% BW per day of RDS delivered via dry-rolled corn. In trial 1, supplementation with RDP had no effect (P > 0.05) on forage DMI, whereas DM digestibility (DMD) by supplemented heifers was greater (P < 0.01) than by unsupplemented heifers. Digestibility of ADF (ADFD) tended to increase (P = 0.07; quadratic effect) with supplemental RDP. In trial 2, forage DMI tended to decrease linearly (P = 0.08) as supplemental NDF increased; however, DMD and ADFD increased linearly (P < 0.01)withsupplemental NDF.In trial 3, unsupplemented heifers had greater (P = 0.02) forage DMI compared with RDS-supplemented heifers. Moreover, DMD increased linearly (P < 0.01) with supplemental NDF. In trial 3, unsupplemented heifers had greater (P =0.02) forage DMI compared with RDS-supplemented heifers. Moreover, DMD increased linearly (P < 0.01) with supplemental starch and was greater (P < 0.01) among starch-supplemented heifers than controls. The ADFD of unsupplemented heifers tended to be greater (P < 0.07) than that of RDS-supplemented heifers. All supplement types positively influenced DM digestion of fescue hay-based diets fed to heifers, whereas only starch negatively affected dietary ADF digestion.