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

Professional Animal Scientist, Dec 2006 by Schmidt, T B, Olson, K C, Linville, M L

Abstract

Holstein heifers (ç = 20; initial BW = 270 ±46 kg) were used in 3 consecutive randomized, complete-block experiments to measure the effects ofruminally degradable protein (RDP), NDF, and ruminalty 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 com. 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

Key words: forage, intake, digestion, supplementation

Introduction

Low-quality forages are marginal for use as the sole ration component for growing heifers or thin dry cows. Therefore, supplementation is used frequently to raise the nutrient content of low-quality, forage-based diets. Supplement composition influences intake and digestion of low-quality forages. Moreover, animal response to certain types of supplementation can be difficult to predict from nutrient density of low-quality forage-based diets. Ruminally degradable protein (RDP) supplements reportedly stimulate intake and digestion of certain low-quality forages (Roster et al., 1996; Bodine et al., 2000). Conversely, starch-based supplements can depress intake and digestion of certain low-quality forages when feeding rates become too great (Olson et al., 1999). Stafford et al. (1996) reported that forage DMI and DM digestion (DMD) by steers increased when they were supplemented with pelleted alfalfa. Moreover, Fieser and Vanzant (2004) reported that digestible OM intake of tall fescue-based diets was greater when steers were fed a fiberbased supplement compared with a starch-based supplement. Further research is needed to establish the potential of protein-, fiber-, and starchbased supplements to improve the feeding value of low-quality forages. Optimal feeding levels for common sources of these nutrients must be identified in order to allow prediction of animal performance with acceptable accuracy. Therefore, a series of studies was conducted to determine the effect of various levels of RDP, fiber, or starch supplementation on intake and digestion of low-quality tall fescue hay by growing heifers.

Materials and Methods

Study Site. The University of Missouri Animal Care and Use Committee approved all animal management practices used in these trials. Twenty Holstein heifers (average initial BW = 270 ± 46 kg) were used in 3 consecutive randomized complete block experiments to measure the effects of RDP, NDF, and ruminally degradable starch (RDS) supplementation on intake and digestibility of low-quality fescue hay by growing heifers. Heifers were housed in a continuously lighted, enclosed barn and restrained in individual tie stalls (1.5 × 2 m) where fresh water was available continuously. Grass hay (8.7% CP, 73.1% NDF, 46.5% ADF), chopped to an approximate particle size of 15-cm, was offered once daily (0700 h) at 150% of the previous 5-d average intake. Hay was harvested in late summer prior to the study from a monoculture stand of Kentucky-31 tall fescue (Festuca arundinacea Schreb.) with an endophyte infection level less than 10%. Supplements were offered daily at 0730 h.

Sampling Periods. Heifers were allowed to adapt to the supplement regime for the first 10 d of each 16-d experiment. Total tract diet digestion was assessed from d 10 to 16 of each experiment: forage samples were collected from d 10 to 15, ort samples were collected from d 11 to 15, and total fecal output was collected from d 12 to 16. Total fecal output was weighed once daily at 0600 h. After thorough mixing, forage, orts, and fecal material were subsampled (approximately 3% of daily total) and weighed. Voluntary forage intake was measured daily during each sampling period. Unshrunk BW of heifers were measured at 0900 h on d 1 and 16 of each experiment. Daily forage, ort, and fecal samples were dried in a forced-air oven (96 h; 100°C) and weighed. Orts and feces were composited within animal and experiment. Daily forage samples were composited across animal and day within experiment.

Supplementation. Prior to experiment 1, heifers were blocked by BW and assigned randomly to receive RDP fed at a daily rate of 0, 0.05, 0.10, 0.15, or 0.20% of BW. Ruminally degradable protein was delivered in the form of soybean meal. It was assumed that soybean meal was 49.9% CP and that the RDP of soybean meal was equal to 65% of CP (NRC, 2000). Feeding rates of soybean meal corresponded to approximately 0.41, 0.82, 1.27, and 1.68 kg/ d (DM basis) for a 272-kg heifer when supplemental RDP was fed at 0.05, 0.10, 0.15, and 0.20% BW, respectively.

At the end of experiment 1, heifers were re-randomized and assigned to one of the following supplemental NDF daily feeding rates: 0, 0.16, 0.33, 0.49, or 0.65% of BW. Supplements were fed on the basis of NDF content as a means of quantifying supplemental fiber intake. Supplemental NDF was delivered in the form of soybean hulls (66.3% NDF). Feeding rates of soybean hulls corresponded to approximately 0.66, 1.35, 2.01, or 2.67 kg/d (DM basis) for a 272-kg heifer when supplemental NDF intake was set at 0.16, 0.33, 0.49, and 0.65% BW, respectively.