Factors affecting intake and digestibility of grazed forage

Abstract

Three different studies were conducted during the 1976, 1977, 1978 growing seasons with cool season annuals to characterize the composition of available and consumed forage under different grazing pressures and to interrelate both plant and animal factors influencing intake and digestibility of grazed forage. In each years study, esophageal and ruminal fistulated animals were used to obtain esophageal forage samples representative of consumed forage. Measurements were also made of standing crop, grazing pressure, chemical composition of available and consumed forage, in vitro and in vivo digestibility, fecal output, and voluntary intake. Estimates of total tract fill and turnover were made by use of rare earth markers in the 1977 and 1978 studies with additional data for ruminal fill and turnover of specific plant parts and particulate dry matter distribution at various sites being determined in the 1978 studies. In the 1976 grazing study, a rotational grazing scheme was developed to observe the effect of frequency and duration of grazing on annual ryegrass forage (Lolium multiflorium). Ten (esophageal and ruminal fistulated) steers were used to graze five series of six plots (3 adjustment and 3 collection) separated by electric fence to give individual plot series of .010, .0151, .0201, .0401, and .0803 ha. Each plot series was grazed by a combination of successive first (A), second (B), and third (C) grazers with two collections (cycle I and II) conducted for each plot series. There was an attempt to have similar initially available forage quantity for each plot series, hence, smaller plots required longer periods of forage growth to obtain similar forage but this forage was more mature. The grazing sequence resulted in the A grazer selecting diets of higher quality than the C grazer with the B grazer being intermediate. The difference in grazer selection were more pronounced with the smaller plots which had older forage. Also, in this study, there was trend for declining in vivo digestibility which corresponded to reduced voluntary intake and a small rise in fecal output. The encountered decreases were more pronounced in cycle II than cycle I, there being smaller differences in chemical composition of plant parts (leaf and stem) in cycle I...