Chickens prefer animal protein diets
Vegetable protein diets are cheaper and safer than animal protein diets but do broiler chickens like them equally? And is their performance the same or better? Australian researchers came up with an answer at the World Poultry Congress in Brazil. Their ?conclusion, as they call it, is a setback for the industry.
By M.M. Bhuiyan, and P.A. Iji, School of Environmental & Rural Science, University of New England, Armidale, NSW, Australia and G. Clatworthy, Inghams Enterprise, Leppington NSW, Sydney, Australia
Cereal grains like wheat, barley, sorghum and maize are the main energy sources used by the Australian poultry industry. Protein and its constituent amino acids are mainly provided by vegetable sources such as soybean, canola, peas, sunflower, and also animal by-products, particularly meat meal. Vegetable protein (VP) sources are typically unbalanced in amino acids although there are significant differences in quality between these sources, soybean being generally regarded as the best of the lot.
The inclusion rate of VP sources may also be limited by the presence of anti-nutritive factors such as phytic acid, enzyme-inhibitors and other compounds that directly affect digestion or nutrient metabolism. Animal by-products like meat meal, meat-and-bone meal, blood meal, fish meal and feather meal are higher in protein content and their amino acids are more balanced than those of plant sources. These by-products are therefore included at up to a level of 10% in traditional poultry diets.
In recent years, the use of animal by-products has come under review and such materials have been banned in the European Union. Poultry tend to benefit the most from the use of animal by-products in the diet, and tend to suffer less directly from some of the zoonotic diseases, unlike cattle and other ruminant species, unless the animal by-product is poorly processed. There has been little research to compare VP diets to traditional diets and results of these are conflicting. There has been more research on the possibilities of replacing more expensive vegetable sources like soybean with alternatives like canola, peanut, sunflower and lupins.
Feeding different protein diets
The experiment was a 2×2×2 factorial design, with diets based on VP or AP; with or without microbial enzymes, and fed to male (initial weight, 44.9±0.9 g) or female (initial weight, 45±0.9 g) chicks. A total of 720 day-old Ross-308 broiler chicks were randomly allocated to eight treatments, each replicated six times with 15 birds in each replicate, on floor pens. Sixty other chicks (mixed-sex) were reared to 14 days, at which point, they were randomly allocated to VP or AP diets, both supplemented with microbial enzymes, each replicated six times. This sub-experiment was conducted to assess feed selection by the chicks. The latter were reared on grower diet and then on finisher diet at 14-21 and 21-28 days of age, respectively. The basal AP diets contained mainly wheat (≤36.0%), sorghum (≤36.5%), millrun (5.0%), meat meal (≤10.0%), soybean meal (≤27.0%), canola meal (4.0%) and cottonseed meal (4.0%) plus fixed minor ingredients. The VP diets contained no millrun and meat meal while mono-dicalcium phosphate was excluded from the AP diets. Both sets of diets were iso-caloric (12.45, 12.55, 12.70 and 12.70 MJ/kg at the starter, grower, finisher and withdrawal phases, respectively). During the same phases, the protein contents were 229.0, 210.0, 194.0 and 181.0 g/kg diet, respectively. The gross responses of the birds in terms of feed intake, growth and feed conversion efficiency were assessed every seven days.
A clear preference
Feed intake to 21 days was reduced by 6% on the VP diets (Table 1). This was increased by enzyme supplementation of the VP diets (1-3%) but reduced on the AP diets. Live weight at 21 days was reduced by 3-4% on the VP diets and increased by the enzyme supplements (4%). The VP diet resulted in a 1% improvement in feed conversion ratio and this was further improved (4%) by enzyme supplements. Flock uniformity at 14 days was better on the AP diets (around 2%) and was increased by enzyme supplementation of the VP diets (3%).
Over the entire feeding period (42 days), feed intake on the VP diets was about 10% lower than on the AP diet but was improved, by 5-6%, through enzyme supplementation (Table 2). Live weight was reduced by up to 6% on the VP diet, but improved (5%) by enzyme supplementation. Feed conversion ratio was better (by 1%) on the VP diets and further improved (2.5%) by enzyme supplements. Flock uniformity at 28 days was better (1%) on VP diets and increased (1%) by enzyme supplements.
Birds generally preferred the AP diet when given a choice (Figure 1). On the grower diet, birds ate 62% of AP and 38% of VP, while on the finisher diets; the selection was 82 and 18%, respectively. Preference was significant in the grower (P<0.01) and finisher (P<0.001) periods.
No sweeping conclusions
The feed intake and body weight gain of birds were lower while on VP based diets than on the AP diet. However, there is an inconsistency in results obtained by different researchers working with different feed formulations, which is a setback for the industry, as it makes it difficult to make sweeping conclusions on the nutritive value of VP diets. The quality of such diets would depend on the basal ingredients that are used in the formulation and also how close the nutrient profiles of the diets are to one another. Often, it is difficult to have such similarities in profiles due to the differences in concentrations of nutrients in the plant protein sources compared to AP sources. The former sources are known to be unbalanced in nutrients, particularly, amino acids. In the present study, the diets were similar in energy, protein, minerals and key amino acids. This is responsible for the similarity in response of the birds on the two sets of diets.
When offered a choice, the birds preferred AP diets to VP diets and no clear reason was established for this preference. The key minerals that would affect feed intake are Na, Cl and Zn. In the present study, there were no marked differences in concentrations of Na and Cl, but slight differences in Zn content. It is not certain if this difference in Zn is large enough to cause the amount of aversion for VP diets that were recorded. Based on the level of performance observed on the VP diets, it is possible that this performance can be further increased, to completely close the gap between VP and AP diets. Such a scenario would be dependent on biotechnical intervention, including some further processing of the plant protein sources, prior to inclusion in the diet or use of appropriate nutrient supplements in the diet.
The microbial enzymes, which were tested in the current study marginally improved performance, particularly on the VP diets but were not effective enough, as to completely close the gap between these diets and the AP diets. It may have been possible, for example to recommend a more appropriate microbial enzyme if the patterns of nutrient digestibility were determined. Litter quality was not different between the two diet groups.
The results of this study have strong implications for the Australian meat chicken industry to utilise broiler diets solely based on vegetable ingredients. The major hindrance to the use of such diets by the industry is feed cost. However, there is an opportunity to develop VP diet formulation and a niche market for broiler products that are labelled as completely raised on VP diet. This product is almost on the same pedestal as free range or organic farming. This could be a new direction to explore the chicken industry.
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