Wheat as a pig feed ingredient
Wheat is often found at prices that compete with maize, but some knowledge is required before formulation with wheat can commence.
Wheat is the main staple in pig diets in several parts of the world (usually in wheat-producing areas), but it remains a mystery for many who do not use it regularly. With cereal prices going through a tremendous instability period for the last decade or so, wheat is often found at prices that compete with maize. In such cases, it is interesting to consider it as an energy source in pig diets. To this end, some knowledge is required before formulation with wheat can commence.
Wheat (also known as corn in United Kingdom, although corn refers to maize in the Americas) is major source of energy containing about 60 percent starch (soft varieties). Major producing countries are China, India, the U.S., Russia, Ukraine and France. Hard varieties (higher in protein, but lower in starch) are cultivated almost exclusively for humans, but off-batches can be traced commercially for use in animal diets. Most information below refers, nevertheless, to soft wheat.
Nutritionally, wheat contains less energy (10.5 MJ NE/kg) than maize, mainly because it contains more protein (10.5 percent) and less starch (60 percent) than maize. Its protein profile is relatively unbalanced, with lysine (0.31 percent) and threonine (0.32 percent) being the first and second limiting amino acids, respectively. Otherwise, wheat has a nutritional profile very similar to that of maize, being low in crude fiber (2.2 percent) and calcium (0.07 percent), but rather higher in phosphorus (0.32 percent). The latter is marginally more digestible (30 percent) than in maize (28 percent).
Wheat contains a substantial fraction of non-starch polysaccharides (approximately 10 percent) that act as anti-nutritional factors limiting its nutritive value. Among them, arabinoxylans (6 percent) are predominant and largely responsible for increased intestinal viscosity in high-wheat diets that may impair overall nutrient digestibility. Non-starch polysaccharides are virtually indigestible by piglets, but they are readily fermented by intestinal microflora producing volatile fatty acids. Pigs fed diets high in wheat tend to produce sticky feces and thus they often appear dirtier compared to pigs fed diets based on maize. All-wheat diets for older pigs rarely cause any problems.
Although there are few, if any, nutritional differences among soft and hard varieties of wheat, the variability in nutritive value among batches even of the same sub-variety is markedly significant. Unfortunately, bushel or hectoliter weight is not correlated to nutritive value, despite common belief. It is rather the concentration of fiber (and especially that determined as neutral-detergent-fiber, NDF) and of course crude protein (CP) that largely determines the nutritive value of wheat. To this effect, the following equation has been proposed to estimate the energy density in wheat (based on Canadian data):
- Digestible Energy (MJ/kg) = 15 + 0.16 x CP (%) – 0.067 x NDF (%), r2 = 0.75
Zijlstra et al. (1999), Canadian Journal of Animal Science 79:187-194.
In wheat, mycotoxin infestation remains a problem throughout the world, especially in low quality batches stored inappropriately. Major mycotoxins affecting wheat include deoxynivalenol, nivalenol and zearalenone, all of which reduce animal performance and impair health. To this end, anti-mycotoxin agents are strongly recommended when contaminated wheat must be fed to pigs, especially to young and breeding stock.
Research has demonstrated that fine grinding of wheat alone cannot be responsible for stomach ulceration, as other stress factors must also be present for such effect. Thus, in pigs with a low susceptibility to ulcers, wheat may be ground down to 600 microns, as with maize. Nevertheless, fine grinding tends to reduce rather drastically the flow characteristics of wheat-based meal diets, and thus, in such cases wheat must be ground to around 800 microns. In contrast, finely-ground wheat is usually considered a rather strong pellet binder (due to its content in gluten), especially in diets containing a moderate amount of fiber (3-5 percent) as this tends to produce an “armed-cement” effect.
Supplementation of wheat-based diets with phytase usually increases phosphorus digestibility (approximately 500 units of phytase can replace 0.1 percent dietary digestible phosphorus), but similar claims regarding amino acid digestibilities remain largely equivocal as with maize-based diets. On the other hand, supplementation with xylanase may enhance energy digestibility in wheat by 2-5 percent and consequently growth performance, but only with medium- and low-quality batches of wheat (that is, those high in fiber and arabinoxylans).
Finally, cooking of wheat is strongly recommended for early piglet diets as this tends to improve its nutritive value and reduce the anti-nutritional effects of non-starch polysaccharides. Wheat should be processed to approximately 70 percent gelatinization, while care should be taken to avoid formation of Maillard reaction products that affect protein quality (usually observed as severe browning or scorching in affected batches). Piglets fed diets based on cooked wheat usually have enhanced performance compared to piglets fed diets based on raw wheat, but such response depends not only on the thermal processing of wheat but also on its overall quality. Clearly, high quality wheat benefits little from interventions.
To avoid possible digestive disturbances from the anti-nutritional properties of wheat, its inclusion level in creep and post-weaning diets is usually limited to 25 percent. Past the first couple weeks post-weaning, however, wheat may be used freely in all pig diets, notwithstanding quality problems that might reduce its upper inclusion level. One exception is made for newly harvested wheat, which must be avoided (if possible) in post-weaning diets and used up to 25 percent in later diets, perhaps with the addition of a potent enzyme; here too, opinions are divided as some nutritionists are not convinced about the "new-wheat" effect -- perhaps due to variety being used.