Forages for pigs: nutritional, physiological and practical implications

Physiological effects of fibrous feeds

Development of the digestive tract

Non-starch polysaccharides have been shown to have a considerable effect on gut size and development, particularly the large intestine. For example Jørgensen et al (1996) found that growing pigs fed a high-fiber diet between 45 and 120 kg had a significantly heavier caecum and colon, and the gut contents were also heavier than in pigs fed a low-fiber diet (Table 2).

The implications of this are a significantly reduced rate of carcass weight gain, compared to live weight gain on fibrous diets, and reduced dressing percentage. Pluske et al (1998) found that carcass yield was reduced by 0.025 units for each additional gramme of fiber intake, and Len et al (unpublished data) reported that dressing % was over two percentage units lower in pigs given a high-fiber diet containing 300 g/kg NDF compared with those given a low-fiber diet (200 g/kg NDF). Another practical implication of the increased weight of the visceral organs is that they have a high rate of energy expenditure, and the gastrointestinal (GI) tract was reported to account for 25% of whole body maintenance, even though it only represents around 4% of body weight (Yen et al 1989). This finding implies that the poorer feed conversion efficiency found when high fiber diets are fed may be partly a result of increased basal heat production, as well as reduced digestibility of energy and nutrients. This increased basal heat production, in combination with increased heat of fermentation in the large intestine, may also reduce feed intake of fibrous diets if the environmental temperature is high, as is often the case in the tropics.

Effects of fibrous forages on digestibility and pig growth

The negative effects of fiber on diet digestibility and pig growth are well documented, and it is generally accepted that diets containing more than 7-10% of fiber result in decreased growth rates (Kass et al 1980). However, the mechanisms involved and the extent of the reduction in nutrient digestibility and growth performance are complex and depend on a number of factors, including for example the fiber source and composition, level of feed and processing method, and the age and breed of the pig. The mechanisms involved in the decrease in diet digestibility, in addition to the presence of the indigestible components in fibrous forages, include the shielding effect of the plant cell contents by the indigestible cell walls, and increased rates of passage of digesta as a result of its increased bulk and water-holding capacity, and also due to irritation of the gut wall mucosa by VFA produced in the hind-gut. In addition reduced feed and energy intake as a result of the presence of anti-nutritional factors, bulkiness and energy dilution, and possibly also heat stress, can also contribute to reduced daily gains.

Effects of fibrous forages on the microflora of the large intestine

Nearly all fiber digestion takes place in the caecum and colon, where bacteria (there are no protozoa in the hind-gut of pigs) break down fermentable carbohydrates (starches, plant cell-wall polysaccharides and host mucopolysaccharides) that have escaped digestion in the stomach and small intestine to produce volatile fatty acids (VFA - mainly acetic, propionic and butyric acids), methane, hydrogen and other gases, and microbial mass (which accounts for around 55% of the faecal DM, more on fibrous diets). VFA from fiber fermentation provide from 5% to 28% of the energy requirements of the growing pig (Kass et al 1980) and the proportion will be even higher for pregnant sows given fibrous diets. However, VFA are not utilized as efficiently as the products of the breakdown of starches and sugars in the small intestine.

Introducing a high-fiber diet initially reduces the microflora of the colon, but then after a few days the number of cellulolytic bacteria increases (Varel 1987). There is also a strong influence of age, and Varel and Pond (1985) calculated that there were 6.7 times more cellulolytic bacteria in the large intestine of mature sows compared to growing pigs when fed the same high fiber diet (containing 40% alfalfa meal) for 3 months, which explains why sows can digest fibrous feeds more efficiently than growing pigs.

Fibrous feeds and gut health

Products high in fermentable dietary fiber are receiving considerable attention in view of their potential positive effects on gut health, and therefore as possible replacements for feed antibiotics. For example, swine dysentery ceased to be a problem on farms when maize silage was fed (Prohaska and Lukacs1984), probably because of the lower gut pH and antibacterial effect of increased VFA production on the organism responsible (S.hyodysenteriae). A similar result was reported by May et al (1994) with a different bacterium (Clostridium difficile). However, Pluske et al (1998) found the opposite effect, and reported that a diet that was high in fermentable carbohydrates predisposed pigs to swine dysentery.

There is also some evidence that a high dietary fiber content can prevent the development of gastric lesions. For example, Henry (1970) showed a reduced incidence of stomach lesions when wood cellulose was added to the diet of growing pigs, although Björklund and Pettersson (1976) found the opposite effect when up to 15% of grass or lucerne (alfalfa) meal was included in pig diets.

Nutritional implications of fibrous feeds

Amino acid balance

Conventional concentrates based on cereals and soybean meal are generally imbalanced, in particular with respect to methionine + cystine and threonine. Most green forages are reasonably high in protein and contain concentrations of the essential amino acids that fairly closely match the pig's requirements. This means that if they are combined with an energy source that is extremely low in protein and fiber, such as sugar cane juice, molasses, sugar palm juice or cassava root meal, then the essential amino acid content of the overall diet will still be balanced and the protein content can be reduced by up to 30-35 % (Speer 1990).

Anti-nutritional factors in forages

Many of the green forages in the tropics that are potentially valuable sources of protein and trace nutrients contain anti-nutritional factors that can depress pig performance, such as cyanogenic glycosides, trypsin inhibitors, mimosine, goitrogens, oxalic acid, tannins and saponins. However many of these can be inactivated to a greater or lesser extent by various processing methods, such as heat treatment, sun-drying and ensiling

Effects on reproduction in sows

Early experiments with green forages suggested that, for example, providing alfalfa meal in the diets of pregnant sows resulted in improved breeding efficiency and fertility, as well as improved viability of the piglets farrowed. The increased litter size in sows given alfalfa or grass meal was found to be due to increased ovulation rate (Teague 1955) and/or pre-natal survival (Mazilis 1962). As the exact mechanisms could not be established it was concluded that green forages provided unknown factors that positively influenced ovulation rates. However, it is more likely that the control diets were deficient in vitamin A or its precursors (carotenes). This is likely to be a particular problem in hot, humid tropical environments, where deterioration of vitamin A in premixes can occur quite rapidly. Although green forages can thus play an important role in ensuring an adequate supply of vitamin A precursors there is also evidence that inclusion of some forages inhibits the absorption of minerals such as calcium, phosphorus and copper. There is also evidence that in addition to the positive effects on fertility through the supply of vitamin A precursors and possible unidentified factors, fibrous feeds as such can improve reproductive performance, as reviewed by Reese (1997) (Table 3).