A possible explanation for limited feed intake of Wheat-based diets by Broilers - By T A Scott, Poultry Research Foundation, University of Sydney - Bioassay research has focused on establishing nutrient levels in grains and their availability to poultry, with a major emphasis on measurement of metabolisable energy (ME). A bioassay, using young broiler chicks incorporates these measures and included measures of variability in nutrient intake and retention (growth and FCR).

Summary

The most perplexing observation from these studies was that feed intake of different wheat-based diets was not associated with determined ME. It was concluded that limitation in intake was dependent on wheat source since these differences in feed intake and subsequent growth were large (>20%). Although processing (pelleting) and xylanase supplementation increased feed intake overall, these treatments did not remove the limitations to feed intake of some wheat-based diets.

A hypothesis was developed that limitation in intake and acquisition of nutrients required for growth was related to differences in digesta passage rate. Digesta passage rate, and ability to obtain nutrients for growth, was improved with either change in the hydration rate of grain by pre-germination or by adding water to the wheat-based diets before feeding. In the latter case, wet-feeding resulted in increased feed intake and growth, however, for some wheat sources the increase in intake was excessive and resulted in marked increases in FCR.

Both pre-germination and wet-feeding activated endogenous wheat enzymes and reduced digesta viscosity. More research is required to determine the factor(s) responsible for limitations in feed intake of wheat-based diets.

Introduction

A broiler chick bioassay has been used extensively to measure the feeding value of different sources of wheat fed in complete diets (80% wheat inclusion) with or without supplemental xylanases (Scott et al., 1998; Scott and Pierce 2001; Scott et al., 2003). Feeding value, based on this bioassay procedure, included measurements of nutrient level, availability, intake and retention (i.e., growth and feed conversion ratio). These studies have raised the serious concern that as yet unidentified factors in wheat (and barley) limit voluntary feed intake, which in turn directly limits growth rate. Within a specific series of bioassays, the between-wheat source variation in feed intake and growth rate has been greater than 20% (Scott 2000).

The variability in feed intake of the various wheat sources was not correlated to measurements of dietary apparent metabolisable energy (AME). This initiated the concern that other factor(s) inherent in these cereals was limiting feed intake and in effect preventing the broiler from attaining a desired nutrient intake to meet its genetic potential for growth (Scott 2000). Xylanase supplementation resulted in an overall increase in feed intake and growth, but the xylanase response varied between wheat source, and supplementation did little to reduce the level of variation in feed intake and growth between wheat sources (Scott 2000). Subsequent studies (Scott 2002; Scott and Silversides 2003) confirmed the variation in voluntary feed intake of different sources of wheat and suggest that there are inherent limitations of all wheat-based diets by broiler chicks and the variation previously reported is only an expression of degree.

There are many poultry bioassay evaluations of feed value of cereals; the predominant emphasis has been to measure nutrient (primarily ME) level and availability or digestibility (Sibbald and Slinger 1962; Schumaier and McGinnis 1967; Davidson et al., 1978; Mollah et al., 1983; and Farrell 1999 for a review). In these bioassays, the bird's preference or ability to consume the diet were either ignored or manipulated to discount variation in feed intake. This was based on the premise that the bird would voluntarily consume a diet to attain a desired energy intake, within reason. This appears to be true for individual ingredients when the diets were diluted, but fails to hold true when different sources of the same ingredient (i.e. different sources of wheat) are compared on an ad libitum basis.

Our observations on limitations in feed intake in effect challenge the strongly held theory that broiler chickens will consume a diet to meet its energy requirements and thereby achieve its genetic potential for growth. Similar observations pertaining to limitations in intake of Australian wheat-based diets have been reported for weanling pigs (Cadogan, 2003). Cadogan was able to explain a large portion of the variation in feed intake to be relative to non-starch carbohydrate fractions of wheat and that xylanase supplementation removed this difference. In broiler chickens, we observed an overall increase in feed intake with xylanase supplmentation, but were not able to remove variation in feed intake between wheat sources with xylanase supplementation (Scott 2000).

Now that our attention has been drawn to the possibility of factor(s) existing in wheat that limit feed intake, a concerted effort has been made to explain this phenomenon. The following presents published and unpublished data on feed intake variability of wheat-based diets and the relationship with growth, AME, and various other parameters measured on the grain and the diet.

Further Information

To continue reading this article, please click here (8 page MSWord Document)

Source: University of Sydney - Published on ThePoultrySite - March 2005