Interaction between carbohydrates and fat in pigs : impact on energy evaluation of feeds

In marketing pigs, nearly 50% of the costs are those of the feed. Therefore, it is necessary to know the nutritional value as accurately as possible.In the Netherlands, pigs receive in general (99%) compound feeds, containing all the nutrients they require. Cereals used to be the major ingredients. However, their proportion was reduced from 40% in 1970 to 15% in the eighties. The use of ingredients other than cereals or tapioca in compound feeds affected the chemical composition of the pig diet: from feeds with a large amount of starch towards feeds containing less starch but more fibrous polysaccharides, that are often called non-starch polysaccharides (NSP). Starch and NSP differ in many aspects: in chemical structure; in the type of nutrients they supply and their effect on other nutrients in the digestion process; efficiency of utilization for energy gain; and other, non-nutritional, aspects.In order to have maximum benefit of their potential nutrient supply, most nutrients need to be digested and absorbed before reaching the terminal ileum. These nutrients are: amino acids from protein, fatty acids from lipids, and glucose from starch and sugars. If they disappear from the large intestine, the energy value of the nutrients will be lower, resulting in a reduced feeding value of the total feed. On the other hand, NSP are fermented mainly in the hindgut, supplying energy to the pig in the form of volatile fatty acids.In general, the energy of pig feeds is evaluated by considering the differential contribution of digestible nutrients to energy supply. Energy evaluation is based on three assumptions: (1) that both the chemical composition and the digestibility coefficients of ingredients in a feed are known and can be derived from feedstuff tables; (2) that the amounts of digestible nutrients in the different ingredients are additive and that there are no interactions between ingredients; and (3) that after digestion the contribution of each nutrient to energy supply is independent of the amounts of other nutrients. Because NSP have a relatively low energy density, they are often supplemented with fat to maintain a certain energy density in the diet. Hence it is assumed that the feeding values are additive. It was found, however, that the combination of NSP rich by-products and fat resulted in less energy gain in pigs than an iso-energetic combination of cereals and fat or by-products separately. It was concluded that the NSP and fat interacted on energy supply to the pigs.It is important to know whether the interactive effect between fat and fermentable carbohydrates takes place prior to the terminal ileum or in the hindgut of the pig. To be able to measure this, a new technique of ileo-cecal cannulation was developed: the steered ileo-cecal valve (SICV). In contrast to other techniques, in this technique the gut remains intact. After testing it with high fibrous diets, it was concluded that both ileal and total tract digestibility can be measured in the same pig. For this, the use of a marker is recommended.To investigate the interaction between NSP and fat, an experiment was performed with 12 diets, in a 4 x 3 factorial arrangement with four amounts of animal fat and three sources of carbohydrate. The amounts of animal fat added to the diets were : 0 (o), 35 (1), 70 (m) and 105 (h) g per kg. The three sources of carbohydrate were maize starch (M), purified cellulose (C) and toasted soya bean hulls (S). The cellulose was. used as a source of poorly fermentable carbohydrate and the soya bean hulls as a source of easily fermentable carbohydrate. These diets were given to pen-housed pigs from 30 to 105 kg live weight, which period is the growing-finishing period in practice. In these pigs both digestibility of the diets and the energy gain were measured. In a separate set of pigs, the digestibility of nutrients at the terminal ileum was measured.All the measured digestibility coefficients were lower than expected from the feedstuff table. This effect was partly attributed to differences in techniques for estimating digestibility between the present experiment (practical conditions) and the experiments supplying data for the feedstuff table. Most of the tabulated values are obtained under well-controlled laboratory conditions. It was found that housing pigs in groups in pens, as in common practice, reduced the digestibility of organic matter with 1.5 %-units, compared to pigs housed in metabolism crates. For protein this difference was larger: on average 3.7 %-units. Feeding high fibrous diets tended to increase these differences.In addition, the assumption of additivity of digestible nutrients in ingredients within a diet was not correct, especially when high fibrous ingredients were used. When cellulose or soya bean hulls were included in the diet, digestibility of protein and fat was worse. It was concluded that intake of dry matter or fibrous material increased endogenous secreted protein with 36.5 g/d per kg NSP consumed, which reduced apparent digestibility. In addition, microbial protein synthesis of 116.6 g/d per kg fermented NSP also reduced apparent protein digestibility. Moreover, the added fat was less digestible at the terminal ileum when combined with cellulose or soya bean hulls in a diet, than when they were all fed separately. The total tract digestibility of the added fat was 91 % with the low fibre diet, but 83% when combined with cellulose and 87% when combined with soya bean hulls.The energy gain predicted from the measured digestible nutrients was compared with the net energy gain as actually achieved. It was concluded that the utilization of energy from fermentable carbohydrates was relatively low: 0.43. This was partly ascribed to energy losses in methane and energy losses in volatile fatty acids in faeces. Of the digestible energy from fermentable carbohydrates 0.81 to 0.90 was available for energy gain in the form of volatile fatty acids. In addition, increased weight of the empty gastrointestinal tract was found, which may have required a large part of the available energy for maintenance, leaving less energy for growth.It is concluded, that digestibility of nutrients should be measured both at the terminal ileum as over the total tract, when large amounts of fermentable carbohydrates are included in the diet. They should be measured under practical conditions. The variation in energy gain between pigs, however, remains relatively large