Characterizating low-quality forage utilization : feed value of a solid stem straw & effects of an abrupt switch from corn-containing to forage-only diets

Abstract

Two separate studies were conducted to characterize low-quality forage utilization. The objective of the first trial was to evaluate the suitability of common small grain straws as efficient roughage sources in livestock diets, specifically comparing a solid-stem winter wheat variety (Bynum) to hollow-stemmed varieties of winter wheat (Norris and Willow Creek) and a barley variety (Geraldine). This was a two-part trial involving an in vitro dry matter disappearance (IVDMD) experiment and an individual lamb feeding trial. The IVDMD kinetics trial was conducted using a Daisy Wheel TM incubator, with replicated straw samples removed after 0, 6, 12, 24 48 and 96 hours. In a feeding trial, 16 crossbred wether lambs were randomly assigned to one of four diets containing chopped straw. The solid-stem characteristic of Bynum did not affect the feeding value of the straw compared with barley straw and other winter wheat varieties. The objective of the second trial was to characterize how quickly the rumen adapts to a forage-only diet after an abrupt switch from a concentrate-containing diet. Twelve ruminally-cannulated Hereford-cross heifers were randomly assigned to 3 individually-fed, pre-experiment diets (4 heifers/diet). Diets were: 1) all forage, 2) 35% concentrate, and 3) 70% concentrate. Heifers were fed the diets for ~100 d before the start of the trial. Pre-experiment diets consisted of grass-alfalfa hay, corn, and soybean meal-urea supplement added to make the diets isonitrogenous at 13% CP. On d 0, diets were abruptly switched to grass hay. In situ digestibility runs were conducted starting on d -8 and ran continuously (d 1, 4, 7, 10, 13, 16, 19, 22) after the diet switch. Duplicate sample bags filled with 5 g of grass hay and a blank bag were incubated for 0, 24, 48, and 96 h. Organic matter and NDF digestibilities in subsequent in situ runs were similar (P > 0.10), regardless of pre-experiment diet. Rate of digestion was not influenced by pre-experiment diet (P = 0.74; avg 4.3 ± 0.002%/h). Forage digestibility was depressed when heifers were fed a high-concentrate diet; however, this effect disappeared within 48 h of feeding 100% forage.