Wheat and barley are important cereal grains, with world production of wheat being over 600 million tonnes and barley production being over 130 million tonnes (USDA, 2008). In the United Kingdom, over 40% of the wheat and barley produced is used in animal feeds and these grains are common sources of energy in diets for growing and finishing pigs. Traditionally, barley has been included at higher inclusion levels due to lower purchase cost and also because of reports that high levels of wheat inclusion (700 g/kg) can cause stomach ulceration (Nielson and Ingvartsen, 2000). Smith and Edwards (1996) reported that wheat-based diets resulted in more gastric ulcers than barley-based diets, and suggested that this may be due to the lower fibre content of wheat, which influences the firmness of the stomach contents (Johansen et al., 1996). Nielsen and Ingvartsen (2000) also found that wheat-based diets (700 g/kg) resulted in a higher incidence of stomach ulceration compared with pigs offered barley-based diets (3.3 vs. 2.0 on scale of 0 to 10).

High levels of wheat inclusion have also been reported to have negative implications for pig carcass grading (Hanrahan and O’Grady, 1984). These workers compared the performance and carcass quality of pigs offered wheat or barley-based diets containing 777 g/kg wheat or 792 g/kg barley. Wheat-based diets resulted in better performance in terms of liveweight gain (LWG), feed:carcass feed conversion ratio (FCR) and kill out percentage. However, backfat depth was increased when the wheat-based diets were offered. Wheat contains a higher digestible energy (DE) content than barley (15.2 vs. 13.9 MJ/kg dry matter (DM)) (Van Barneveld, 1999). However, the effect of wheat inclusion on carcass quality and stomach ulceration requires further investigation and the primary aim of this study was to examine the effect of increasing the level of wheat inclusion in diets offered to growing/finishing pigs.

Four experimental diets were formulated to contain 700 g/kg cereal (Table 1). Diet 1 contained 700 g/kg wheat, diet 2 contained 600 g/kg wheat and 100 g/kg barley, diet 3 contained 500 g/kg wheat and 200 g/kg barley and diet 4 contained 400 g/kg wheat and 300 g/kg barley. Diets were mixed and cold pelleted on site. Cereals were ground using a hammer mill (screen size 5 mm) and raw materials were weighed prior to mixing in a Bental mixer. Diets were cold pelleted in a Lister 7.5 cuber using a 3 mm die, cooled, stored in sealed 25 kg bags, labelled and transferred to a cool, dry environment. The chemical composition and gross energy contents of the diets are presented in Table 1. Diets contained similar levels of crude protein, oil and gross energy but levels of fibre and ash increased with decreasing wheat inclusion.

The DE content of the diets increased as the inclusion of wheat increased and this was due to the higher level of DE in wheat, when compared with barley (Van Barneveld et al., 1999). On the whole, there were no significant differences in feed intake as a result of wheat inclusion, although there was a trend for pigs offered the diets containing higher levels of wheat to eat less (especially in individually housed pigs). This is in keeping with the results of Weatherup et al. (2002). These workers found that pigs housed individually were able to increase feed intake as DE was reduced, which indicated that pigs housed individually attempted to eat to a constant energy intake. However, it must be highlighted that pigs offered the 600 g/kg wheat diet had numerically higher feed intakes over the entire experimental period. The differences in feed intake between individually and group housed pigs reported by Weatherup et al. (2002) was observed within the current study. Over the period of 10–15 weeks of age, individually housed pigs consumed on average 379 g/d more feed than those housed in groups. This higher feed intake was reflected in increased LWG with individually housed pigs growing at a faster rate (958 vs. 729 g/d). There was no significant effect of level of wheat inclusion on carcass parameters, although wheat at the higher levels of inclusion tended to increase backfat depth at the P₂ position, which could lead to increased grading penalties in a commercial situation. Hanrahan and O’Grady (1984) also reported that wheat resulted in greater backfat depth when compared to barley-based diets and this may be attributed to the lower level of dietary lysine as a proportion of DE in the wheat-based diets. However, when the wheat-based diets were supplemented with additional lysine, the effect on backfat was still apparent indicating that the utilization of wheat DE was less efficient than that of barley DE. In the current study there was no significant difference in FCR and no effect of wheat inclusion on kill out percentage indicating that the utilization of energy from barley and wheat was equal (Cole et al., 1972).

The level of stomach ulceration was low across all the treatments (<1.01 on scale of 0–10) (Nielsen and Ingvartsen, 2000) and increasing the level of wheat inclusion had no significant effect. This finding is in contrast to Smith and Edwards (1996) who found that ulceration score was higher for wheat compared with barley-based diets (3.73 cf 2.16, p<0.001) (level of ulceration was scored using a modification of the method described by Potkins et al., 1989). These workers concluded that there was a ‘factor’ in the wheat which contributed to ulceration. This ‘factor’ may be the lower fibre content of wheat which results in more fluid stomach contents (Johansen et al., 1996). However, the pelleting process may have negated the difference between wheat and barley. Pelleting has been shown to increase keratinisation and ulceration (Wonda et al., 1992) although other workers have reported no negative effect of pelleting (Chae et al., 1899). It was expected that stomach weights for pigs offered barley-based diets would be heavier than those offered wheat-based diets in accord with previous work reported by Nielsen and Ingvartsen (2000). However, stomach weight was not significantly different in this study and the small intestine weight was highest for the diet containing the highest level of wheat. This may be a result of the increased digestibility with the higher levels of wheat inclusion, which contributed to the development of the small intestine (Morel and Cottam, 2007). The digestibility coefficients followed the expected pattern with digestibility and DE content increasing linearly with increasing wheat content. This effect can be attributed to the chemical composition of the cereals, with wheat containing a higher level of starch and a lower level of fibre than barley (Bach Knudsen, 1997).

While increasing level of wheat inclusion increased dietary DE content and nutrient digestibility, there was no effect on performance, although individually housed pigs tended to reduce intake and attempted to eat to a constant energy intake. None of the treatments resulted in high levels of stomach ulceration and there was no effect of increasing wheat level. However, higher dietary levels of wheat tended to increase backfat depth which could have negative grading implications although more research is required to fully elucidate the effect of high wheat levels on carcass quality.