Processing index of barley grain and dietary undigested neutral detergent fiber concentration affected chewing behavior, ruminal pH, and total tract nutrient digestibility of heifers fed a high-grain diet

As a strategy to improve gain-to-feed, feedlot cattle are often fed diets containing a high proportion of grain with minimal forage. However, diets with insufficient fibre may predispose cattle to ruminal and hind-gut acidosis, liver abscesses. This may suggest that there could be a minimal fibre requirement. It is likely that numerous factors affect the minimal fibre requirement including fermentability of the feeds, forage inclusion rate, forage particle size, starch source and processing, and many others. Thus, adequate characterization of dietary parameters may help to define the fibre requirement. A parameter, namely undegradable NDF (uNDF) recently incorporated into some nutrition model to predict rumen pH. We hypothesize that the uNDF will have a direct relationship on rumen motility, chewing activity, and hence rumen function. Furthermore, both the dietary uNDF and ruminally fermentable carbohydrate are primary factors affecting the minimum fibre required for finishing cattle.

A study with overall goal to evaluate whether processing index (PI; weight after processing/weight before processing × 100) of barley grain (i.e., fermentability in the rumen) and dietary uNDF concentration affects the requirement for fibre of finishing beef cattle. The specific objectives are to determine the effect of barley processing and dietary uNDF content on feed intake, chewing activity, rumen pH and fermentation parameters, total tact digestibility, total tract barrier function, and blood metabolites. Six ruminally cannulated beef heifers were used in a 6 × 6 Latin square design with 3 PI (65, 75 and 85%; fine, medium, coarse, respectively) × 2 uNDF levels (low and high; 4.6 vs. 5.6% of dry basis) factorial arrangement. The heifers were fed ad libitum a total mixed ration consisting of 10% barley silage (low uNDF), or 5% silage and 5% straw (high uNDF), 87% dry-rolled barley grain, and 3% vitamin and mineral supplement.

Numerous interactions between PI of barley and dietary uNDF concentration were observed (feed intake, rumination, and total tract digestibility of nutrients), suggesting the effects of these variables were interdependent for beef heifers fed high grain diets. Overall, increasing extent of barley processing tended to decrease ruminating activity of cattle fed low uNDF diets, and increased the total tract digestibility of organic matter, starch and protein without impacting ruminal pH and fermentation parameters. We conclude that a PI of 75% is optimum in terms of feed digestibility because further decreasing the PI to 65% did not result in a further improvement in digestibility. Increasing dietary uNDF concentration elevated intake of fiber regardless of PI of barley grain, and stimulated rumination when barley was processed with a PI of 65%. The ruminal pH status improved with high uNDF compared with low uNDF diets. Thus, increasing the uNDF concentration of diets is an effective strategy to improve ruminal pH status in finishing cattle without increasing the proportion of forage in the diet, regardless of the extent of grain processing. In contrast, minimizing the extent of grain processing did not reduce the risk of rumen acidosis.