Nutrition and intramuscular fat: Effect of the levels of protein, lysine and other amino acids

Another feeding strategy for increasing the IMF content consists in reducing the supply of protein and/or certain amino acids as, for instance, lysine, relative to the energy supply of the diets. Australian and some American studies have shown that when reducing the protein and/or lysine supply in the diet the IMF content increased significantly in the loin and the ham. Although the production results were not affected, they saw an increase in the backfat thickness or a drop in the carcass weight or the loin eye area when reducing the level of lysine in the diet. Nevertheless, in a study carried out in collaboration with the INRA some years ago, we see that when reducing the crude protein content but supplementing with synthetic amino acids and adding some more starch or starch and fat for maintaining the energy level and the lysine:energy ratios of the diets, the IMF content of the semimembranosus muscle (ham) was significantly increased in castrated pigs housed at thermoneutrality or very high temperature conditions. Also, no differences were seen in the percentage of lean tissue, the backfat thickness, or the bacon fat or the kidney area fat weight.

Recently, we have carried out a trial to study what would happen when reducing the protein and lysine levels in the compound feed for the finishing of Landrace x Duroc castrated pigs fattened until 115 kg, maintaining the net energy content level of the diets constant (Table 3). When reducing protein without reducing lysine the production results are maintained or even improved. Also, the IMF content in the loin or the ham is increased without worsening the carcass quality. When reducing lysine but maintaining the protein level the IMF content is also increased. Nevertheless, the production results and the carcass quality worsen significantly. Reducing lysine and protein at the same time not only allows the increasing of the IMF, but it also worsens the carcass quality and the production results. In this experiment, the net energy was kept constant: in the case of having chosen to maintain the lysine:net energy ratio, a clear reduction of the growth, an increase in the number of days needed to reach the slaughter weight, a general fat deposition in the animal, the carcass and maybe the meat would have been expected. Summarizing, it seems that the reduction in the crude protein or lysine content, but not of the two of them at the same time, could allow to increase the IMF content, but the lysine levels or its ratio with respect to the net energy level must be watched closely in order not to worsen other production parameters. In the same way, in these cases it will be fundamental to know the protein growth potential of the genetic type of the pigs that we are working with, as well as their corresponding voluntary food consumption curve in order to proceed to the most convenient adjustments.

Table 3. Influence of the level of protein and lysine in the diet on the IMF content of the loin and the ham.

Finally, there are a few studies that show that a supplementation with certain amino acids, as leucine (LEU) or arginine (ARG) could be useful to increase the IMF content. Of course, in the same experiments in which the diets were supplemented with leucine, the impact on the IMF content was lower than the reduction of the lysine level and/or the use of diets deficient in several amino acids. Nevertheless, in these same studies, the supplementation of the diets with leucine also affected the production parameters negatively. In relation to arginine, an amino acid that seems to have a body energy distribution effect, there are signals that it could increase the lipid content in the loin and the ham. In order to check the effect of the supplementation with both amino acids, our group has carried out a factorial experiment (2x2) during the finishing stage using Pietrain x (Landrace x Large White) castrated pigs. In general, no differences in the production parameters due to the use of the experimental diets were seen. The leucine supplementation did not affect any of the quality parameters, and it did not even increase the IMF content, whilst the arginine supplementation seemed to reduce the backfat thickness, improving the lean meat percentage of the carcass. Nevertheless, the effect on the IMF content of the loin was negative.

Summarizing, and at least under similar conditions to those of our experimental tests, it does not seem that the supplementation with these amino acids (LEU, ARG) can be a recommendable strategy when the goal is to increase the IMF content and, so, the organoleptic (taste and sensory) traits of pig meat.

Table 4. Influence of the supplementation with leucine and/or arginine on the production parameters and the IMF content in finishing pigs.

Finally, and in order to summarize, we could say that it is not easy to modify the IMF content through the diet, especially due to the risk of causing an adverse effect on other production parameters. Nevertheless, the reduction in the protein level seems to be a promising strategy, provided that the lysine level is adapted to the kind of pig used and that the rest of the amino acids are balanced according to the ideal protein profile.