SOME PRINCIPLES OF PIG NUTRITION IN THE TROPICS
Danilo A. Pezo. Animal Nutritionist. ILRI-SEA. c/o IRRI, Los Banos, Laguna, Philippines
Small-scale rural farmers cannot compete with the large-scale, peri-urban, and vertically integrated systems using the same type of feed resources (i.e. imported grain cereals and protein sources), animal genotypes and facilities. In order to succeed, these farmers need to develop alternative pig production systems, mainly based on the use of locally available feed resources, preferably produced in the same farm, and avoiding as much as possible any processing that will increase the costs of production. However, in order to make an efficient use of those resources, it is necessary to understand the principles that govern swine nutrition, as well as the impact of using feed resources with generally low dry matter content, and rich in sugars, lipids and/or fiber.
The Digestive Tract
The gastrointestinal tract of the pig is relatively simple. It has three compartments: the stomach, the small intestine and the large intestine (Figure 1). The stomach is a reservoir where ingested feed is mixed and subject to the action of pepsin to partially degrade the diet proteins. In the small intestine not only occurs the enzymatic digestion of sugars, starch, lipids and protein, but also the absorption of the products of digestion (glucose, fatty acids and amino acids). The large intestine is the fermentation compartment, where microorganisms (intestinal flora) attack the feed components that have not been digested in the previous compartments (including the fiber), intestinal secretions and cell particles falling off from the mucosa that linens the small intestine. Absorption of part of the products of fermentation, water and minerals also occur in the large intestine.
While the fermentation ability of the pig in its large intestine is not comparable to that in the rumen-reticulum of ruminants (Figure 2), it is frequently underestimated. The volatile fatty acids produced during fermentation in the large intestine of the pig can supply up to 30% of the energy requirements of growing/finishing animals.
The digesta in all compartments is liquid, except in the last portion of the large intestine (the rectum) where massive absorption of water occurs, making the feces more solid. The length of time that the digesta is retained in the large intestine determines the entire transit time along the digestive tract. The inclusion of fibrous feeds in the ration (i.e., sweet potato vines) results in increased retention time in the large intestine, and consequently in the small intestine, giving better chance to the bile and enzymes secreted by the pancreas to digest the feed that reaches the small intestine. This also favors absorption.
The digestive capacity of the pig increases with age. Lactating pigs
have a well developed stomach to cloth and digest milk, but during the
first weeks of age the small intestine and the pancreas develop, favoring
the use of solid starchy and protein feeds. The maturation of the large
intestine is slower, therefore it is better to delay the inclusion of
fibrous feeds in rations, using these mostly for growing/ finishing and
Figure 2. Gastro intestinal tract of a sheep
While industrial systems aim to obtain the maximum biological performance of animals, this should not be the goal for small-scale pig production systems in the tropics. The law of diminishing returns applies to animal feeding, as it does in other biological processes. Therefore, the increments in response require each time higher levels of inputs, and it is more critical when approaching the maximum. Swine production based on non-traditional feed resources that are mostly energy-rich creates a greater dependence on high quality protein sources, which are expensive and difficult to be produced at the farm level.
Although energy and protein requirements for pigs have been already developed in temperate areas, these need to be revised under tropical conditions, not only considering the type of feed resources available, but also the impact that temperature and relative humidity have on feed intake and heat load dissipation. Higher ambient temperature results in lower feed intake, therefore diet composition needs to be adjusted to provide the nutrients required, trying to maintain an adequate protein-energy ratio as well.
Frequently, energy and protein requirements for pigs are considered separately, but the balance between these is critical on the efficiency of feed conversion. Although in practical terms we refer to protein requirements for pigs, there are specific needs for essential amino acids, with lysine, methionine and tryptophan being the most limiting under ordinary farm conditions.
Most tubers, roots, fruits and other non-traditional feed resources used for pig feeding in small scale systems in the tropics, including sweet potato tubers, are rich in energy but have much lower crude protein content than grains (1.5-5% vs. 8-10%, respectively). Although some may consider this a disadvantage, in fact this could be the contrary, as it is easier to find a protein supplement already balanced for essential amino acids, rather than try to compensate for the imbalanced amino acids in the basal diet, as in the case of grain-based rations.
In general it has been suggested that a growing/finishing pig, weighing from 25 to 90 kg, and fed tropical roots, tubers or sugar cane, requires 2.5 to 2.8 kg of dry matter (DM) per day. These rations generally consist of 2.0 to 2.3 kg of DM of the energy source and 500g of a protein supplement. Since the supplement is about one-fifth of the total intake, then vitamins and minerals should be included at five times the concentration recommended for grain-based concentrates.
Supplementation of Sweet Potato-based Diets
Sweet-potato tubers are rich sources of starch, vitamin A, vitamin C and several vitamins of the B complex (niacin, thiamine and riboflavin), but poor in protein and fat, and low in fiber. As indicated before, to obtain adequate live weight gain responses, it is necessary to have a balance between amino acid and energy contents in the diet; therefore, diets based on the use of sweet potato tubers need to be accordingly supplemented with protein sources.
Another aspect to be considered when sweet potatoes are fed to pigs is that raw tubers have trypsin inhibitors that interfere with protein digestibility, resulting in poorer live weight gains of growing/finishing pigs. Cooking not only reduces those compounds, but also increases starch digestibility. Ensiling can lower the anti-trypsin factors, but do not eliminate them, therefore protein digestibility in ensiled sweet potato is intermediate between fresh and cooked. Sun drying of sweet potato chips also reduces the presence of trypsin inhibitors to levels equivalent to the ones obtained by cooking. Also, peeling sweet potatoes significantly increases their protein digestibility. However, it does not have an effect on the digestible or metabolizable energy values of sweet potato tubers.
Sweet potato vines are rich in protein, sugars, vitamins, but are relatively high in fiber and have a very low dry matter content. The last two factors may explain why the introduction of sweet potato vines in the diet usually results in reduced dry mater intake. Sweet potato vines can be used to partially replace other protein sources (e.g., soybean meal), but it should not exceed 13% of the total ration in growing/finishing pig diets. The use of sweet potato vines could be higher in diets for mature females and males (reproductive animals), as in those cases the interest is on maintaining rather than gaining weight, besides the fact that in those animals the large intestine has reached maximum development.
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