Poor uniformity in a broiler operation reduces revenue and increases waste. Uniformity in body weight at harvest is influenced by variation in genotype, environment and feed composition and form. Variation in growth in broilers within each sex is usually relatively small, but increases markedly when a poor quality feed is given. In attempting to grow to meet their potential when fed a diet low in protein, birds need to overconsume energy and then release excessive amounts of heat to the environment, although this ability is constrained by both feather cover and the ability to fatten. Consequently, as broiler genotypes have become faster growing and leaner, there is an increased need to feed higher levels of balanced protein in a cooler environment as a means of improving uniformity. Separating the sexes and reducing the range in day-old body weights will assist in achieving better uniformity at harvest.

Currently whole grain is incorporated into poultry diets to lower production costs, stimulate the development of the gizzard, prevent potentially pathogenic bacteria from entering the intestine and improve the growth rate and feed conversion of birds. The present review discusses the results of experiments performed on turkeys where whole grain (WG) or large-sized feed particles were incorporated into diets with the use of different processing methods (with pre- or post-pelleting inclusion) and in different quantities. The results of recent experiments indicated that, within the first month, gizzard weight increased significantly when the size of feed particles was increased from 380 to 806 mm, or when turkey diets were supplemented with 20% WG. In older birds, gizzard weight increased when the WG content of pelleted diets exceeded 20%. Dietary dilution with WG did not cause significant changes in the development or functionality of the small intestine, including mucosal enzyme activity. The presence of WG in the diet increased the amount of coarse particles in the digesta leaving the gizzard, which did not lower the apparent digestibility of dry matter and crude fibre or nitrogen retention. The inclusion of WG in turkey diets led to a decrease in the counts of Clostridium spp., Campylobacter and/or Escherichia coli, and an increase in the counts of Lactobacillus spp. and/or Bifidobacterium spp. in the caecal digesta. However the effect of WG inclusion in turkey diets on gut morphology has been investigated in only a few studies to date, and the results are inconclusive.

This review highlights the limited amount of research conducted regarding the optimum particle size of different feeds for efficient poultry production. The current industry practice of using highly processed, pelleted diets masks the influence of particle size, but some reports suggest that the effects of feed particle size on performance may be maintained even after pelleting. There appears to be a general consensus that particle sizes of broiler diets based on maize or sorghum, optimum particle size should be between 600 and 900 μm. Available data clearly show that grain particle size is more critical in mash diets than in pelleted or crumble diets. Although it has been postulated that finer grinding increases substrate availability for enzymatic digestion, there is evidence that coarser grinding to a more uniform particle size improves the performance of birds maintained on mash diets. This counter-intuitive effect may result from the positive effect of feed particle size on gizzard development. A more developed gizzard is associated with increased grinding activity, resulting in increased gut motility and greater digestion of nutrients. Although grinding to fine particle size is thought to improve pellet quality, it will markedly increase energy consumption during milling. Systematic investigations on the relationships of feed particle size and diet uniformity with bird performance, gut health and pellet quality are warranted if efficiency is to be optimised in respect of the energy expenditure of grinding.