The digestible energy value of wheat for pigs, with special reference to the post-weaned animal [Review]

https://doi.org/10.1016/j.anifeedsci.2005年02月02日2 Get rights and content

Abstract

Wheat used in pig diets has variable physical characteristics, such as bushel weight and contains variable quantities of nutrients and anti-nutritional factors due mainly to the genetic and environmental influences the crop encounters during growth. However, variations in physical and chemical characteristics are not generally considered when wheat is used in the formulation of pig diets. This review aims to illustrate the variation in wheat quality that occurs, with most focus on the digestible energy (DE) content since the major purpose of wheat in pig diets is as an energy supply, and to address factors influencing the variation of energy value, with special reference to weaner pigs. The factors discussed in this review include physical and chemical characteristics, variety, growing region, season, post-harvest storage, the use of supplemental enzymes, such as xylanase and phytase, age of pigs and processing of grains.

Introduction

Wheat is a major cereal grain used in weaner pig diets in some countries due to its generally lower cost per megajoule of digestible energy (DE) and its relatively high protein content in comparison to other grains. Since wheat is used in pig diets predominately as an energy source, the DE content of this grain has been studied to improve the accuracy of feed formulation, with the overriding majority of estimates having been determined in growing-finishing pigs. However, the energy content of wheat in diets for young pigs has received less attention, and has generally been assumed to have the same nutritive value as is found in larger pigs. Nutritionists, therefore, use tabulated DE values, traditionally derived from growing pigs, in the formulation of diets ranging from weanling pigs through to finishing pigs. Due to the underdeveloped gastrointestinal tract (Pluske et al., 2003), immature endogenous enzyme systems (Cranwell, 1995) and low population numbers and diversity of microflora in the gut of young pigs compared to that for older pigs (Fernandez et al., 1986), it is possible that the energy digestibility of specific ingredients might differ in pigs of various ages, especially in young pigs. Moreover, many other factors, such as the wheat variety, growing locality, climate, post-harvest storage, exogenous enzyme supplementation, grain processing and carbohydrate characteristics may contribute to variation in wheat DE content (van Barneveld, 1997). Therefore, for optimal energy utilisation of wheat as a feed grain, there is a need to quantify the variation in DE content and to understand the interacting factors that influence energy digestibility. Investigations of wheat quality in growing pigs have shown a large variation in DE content (up to 3.7 MJ/kg DM; see van Barneveld, 1999) due to wheat variety and growing conditions, such as locality and the level of precipitation received during growth of the crop. Such wide variation in wheat energy value is of importance to the feed manufacturing industry because precise formulation and optimal utilisation of feed ingredients are the key factors in terms of the efficient conversion of feed to meat. This review will investigate the various factors that influence the energy value of wheat as an ingredient in pig diets, especially for the weanling pig, where an argument could be made for the use of different DE values to those established in older pigs. The review will discuss the physical and chemical characteristics of wheat that influence its nutritive value, particularly energy, for young pigs, and attempt to establish broad associations to assist the feed manufacturing industry in the prediction of wheat energy value.

Section snippets

Chemical composition of wheat

Starch, dietary fibre (DF) (non-starch polysaccharides, or NSP, plus lignin) and protein are the main components of wheat grain (Table 1). Wheat contains 50% more lysine and three times more tryptophan than maize and has about the same amino acid profiles as barley. However, wheat has 15–20 g fat/kg compared to 30–40 g fat/kg for corn, which partially contributes to the lower energy content compared with maize (Mavromichalis and Hancock, 1999). In addition, wheat contains low-molecular-weight

Factors influencing the DE variation of wheat

The DE content of wheat from reports in the literature is presented in Table 3, and ranges from 13.3 to 17.0 MJ/kg DM. The extraction of available energy from a given wheat by pigs is not constant due mainly to its variable chemical composition, and this has been reported previously in the literature with wheat-based diets (Anderson and Bell, 1983b). The utilisation of energy by non-ruminant animals is known to be affected by the digestible energy content of wheat (Rogel et al., 1987, i.e.,

Prediction of DE content

To overcome the considerable physical, labour and cost requirements of estimating wheat DE content by faecal collection, numerous attempts have been made to rapidly predict DE content based on physical and chemical characteristics. An early attempt to predict the DE content from 21 feed ingredients (e.g., cereals, oil-seed meals, animal proteins and fibre sources) used in pig diets was made by Morgan et al. (1975), and the authors suggested that total digestible nutrients (TDN), which is a sum

Net energy (NE) value of wheat and relationship to DE

The advantages of net energy (NE) over DE or ME have been documented (Noblet and Henry, 1991), since the NE takes into account the energy losses during the digestive and metabolic processes. The NE also takes into account any differences in energy inputs into the process of digestion, such as pancreatic secretions, mucin production and increased enterocyte turnover. Such inputs vary and can be ingredient dependent, which may explain different NE:ME ratios when fed to pigs. Just et al. (1983)

Conclusion

The chemical and nutritional variation of wheat was reviewed with possible factors influencing energy values discussed, with special reference to weaner pigs. It is apparent that wheat variety and the growing environment, e.g., soil types, agronomic practices, precipitation level and the growing season can influence the chemical composition of wheat. The extent of variation in chemical composition of wheat can then be an indicator of the nutritive value of wheat, such as the DE content. The

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