Current use of rice products in animal diets

By: Karel Thurman

Adapted to growth in a variety of regions worldwide, rice (Oryza sativa L) consists of over 120,000 cultivars. Although the shape of the rice kernel and the amylose/amylopectin ratio may strongly vary, all rice varieties share the same high nutritional value (Table 1). As in other cereals the endosperm of rice is rich in starch (up to 80%). Compared to more conventional cereals, rice has exceptionally small starch granules. This explains the excellent digestibility of rice (>97%) and the low levels of undegradable starch.

Rice is not only very nutritious but also hypo-allergenic due to the lack of gluten. The protein fraction is characterised by excellent digestibility (98%) and a very complete, balanced amino acid profile (Table 2). Rice oil is recognised for its high levels of unsaturated fatty acids and contains high levels of vitamin E, the well-known natural anti-oxidant.

Rice is naturally enriched in a number of sometimes unknown nutritious factors and benefits from the absence of gluten or other allergenic components. This makes it the ideal cereal for sensitive species and animal categories.

Rich source of nutrients

When rice is harvested, the kernel is still enclosed in a fibrous husk. This husk is quickly removed after harvesting and can serve as bedding (Figure 1). The brown rice is then polished in order to separate the white endosperm from the outer fibrous layers and the germ. Rice germ is rich in fat, with particularly high levels of unsaturated fatty acids. It also contains high levels of vitamins, minerals, anti-oxidants, and phytosterols (γ-oryzanol), which explains the high nutritional value of this fraction of the rice kernel. The rice germ in combination with the outer layers is stabilised via heat treatment and this “rice bran” comprises the best of the rice in condensed form.

After polishing, the remaining fraction is white rice which is widely known and consumed as such. It mainly consists of endosperm, is very low in fibre and contains high levels of readily digestible starches. Rice starch is characterised by the small granular structure (2-8 µm). This facilitates the amylase enzymes, which can act on a larger contact surface than, for example, potato (15-80 µm) or wheat starch (3-40 µm). Enzymatic breakdown in the intestinal tract is enhanced, which explains why rice is so easily digested. However, the amylose/amylopectin ratio may vary considerably among the various rice cultivars. Increased amylose content means slower starch digestion, resulting in prolonged energy release.

Rice for young animals

Because of the absence of anti-nutritional factors (ANFs) and the high digestibility of the various components, rice and its derivatives are very useful for young animals. The high levels of easily accessible energy can help young animals to recover more quickly after stressful events such as weaning. Various authors (Mateoset al., 2006; Vicenteet al., 2008) have reported the benefits of the inclusion of rice in young piglets’ diets, with effects ranging from improved intestinal health to better zootechnical performance.

The high biological value of rice protein explains why rice protein concentrate (RPC) can be successfully used in diets for young piglets. Its excellent digestibility and neutral taste supports inclusion of RPC in diets for these animals rather than whey protein or soy protein concentrate. Compared to soy protein concentrate, digestion of RPC does not require extensive adaptations in the enzymatic system, which explains why pigs benefit from it, especially during the first week after weaning.

Chinese research (Zheng Pinget al., 2008) concludes that up to 10% RPC may be used to replace dried whey in the diet of early-weaned piglets, and this can improve their growth performances. Rice has always been considered to have a nutraceutical functioning in alleviating the incidence of diarrhoea and reducing dehydration. Recent work by Montagneet al.(2004), for example, indicates that the resistance of piglets against post-weaning colibacillosis diarrhoea (PWC) is increased when rice is included in the diet.

The study concludes that a weaning diet based on rice results in drier faeces and lower E. coliscores compared with wheat-based diets. The mechanism how rice improves health has not yet been explained, but in a recent Japanese study, Yamagishiet al.(2008), reveals that rice contains specific polysaccharides which may stimulate the Complement System and, in that sense, have a immuno-stimulating function. It is however certain that this “Unknown Health Factor” (UHF) has its benefits in feeding animals that have to cope with digestive stress, and that inclusion of rice in a diet helps the animals to adapt to the disease challenges of the environment. Table 3 shows a study by Vicenteet al.(2008) in which rice and corn were compared in piglets.

Pets, horses and fish

Compared to other animal categories, rice already has an excellent reputation in pet food applications.
The typical taste and excellent digestibility of these products makes rice an ingredient of choice for a number of premium products. Rice products contain no gluten, and the high safety of this food-grade cereal meets the highest standard qualities of this particular industry. Rice protein is, because of its high biological value, also often used in petfood as a source of protein to replace animal protein. Stabilised rice bran including high levels of rice oil is also an interesting ingredient for pet nutrition.

The high levels of vitamins and anti-oxidants stimulate good health and results in a better general condition of the animal. Rice bran is a beneficial ingredient for equine nutrition. Providing energy via fat combined with high levels of fibre makes it a perfect ingredient for the particular digestive system of these animals. It is also a palatable ingredient, greatly appreciated by the animals. The high levels of vitamins and minerals contribute to the general health and well-being of the horse; this has been confirmed in research by Coombset al.(1997).

The booming aquafeed business is placing incremental stress on global fish meal supplies. Nutritionists are increasingly looking to vegetable protein sources to replace this expensive and rare ingredient and to develop a more sustainable aquafeed production, alleviating the environmental consequences of industrial aquaculture.
Studies on seabream (Palmegianoet al.,2007) and rainbow trout (Palmegianoet al., 2006) indicate that rice protein concentrate is a good alternative to fish meal; inclusion up to 20% is feasible without negative influence on zootechnical performance or fillet quality. Not only rice protein, but also rice starches and flours may be included in aquafeed. The specific structure of rice starch may be beneficial for a number of technological parameters important for aquafeed.

Rice starch, an extrusion enhancer

Rice is valuable not only for its nutritional values but also merits consideration for its technological characteristics. Rice starch is characterised by a very low amylopectin chain count (DP12-22) of intermediate degree of polymerisation. Rice amylose is also characterised by a very high degree of branching, inhibiting amylase diffusion from the starch granules.

These specific characteristics give favourable behaviour during extrusion processes. The high hotset temperature ensures the optimum porosity and smooth surface of the extrudate. These characteristics imply that rice flours and starches can be used to fine-tune the physical characteristics of the extrusion process, leading to optimum quality of the end-product. As extrusion continues to become increasingly important in the production of animal feed, these technological benefits should not be underestimated.

Conclusion

Despite the fact that rice is not widely used in European animal industry, a variety of studies clearly show the benefits of including rice and its derivatives in a number of applications. Rice derivatives may be the ingredients of choice; not only for nutritional reasons but also because of their technological characteristics.

References are available on request

 Source: Feed Mix magazine Vol. 17 No. 1 (2009)