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The relationship between tryptophan and swine behavior

Nutrition, performance and animal welfare are intertwined. For example, minor changes in tryptophan levels influence swine behavior.

It’s no secret that nutrition and proper diet formulation are hugely important for animal agriculture. It’s also true that ensuring optimal nutrition presents a wide variety of challenges. Diets must be formulated to contain the correct levels and ratios of nutrients, and must be appropriate for each stage of production. Additionally, consistent and quality ingredients are necessary to support maximal production of meat, milk and eggs. Finally, input costs must be controlled for the overall operation to meet with success.

In addition to focusing on growth performance and operational efficiency, it is important to consider how diet formulation affects other aspects of production, such as animal behavior and welfare. All of these are related and are affected by the smallest of changes in nutrient levels.

Given the complexities involved with protein production, it’s important to hone in on the role that nutrition plays in ensuring animal welfare within any successful management program.

Nutrients do many things

Nutrients play roles beyond supporting the growth of meat tissue, and include the production of neurotransmitters in the brain that affect behavior.

Tryptophan (TRP) is an essential amino acid in protein that is converted to serotonin, a potent neurotransmitter that controls behavior. The uptake of TRP from circulation in the blood to the brain is almost never saturated.

Simply put, more digestible TRP in the diet leads to more TRP in the blood, which increases TRP in the brain. And, the increased TRP in the brain is converted to more serotonin. Thus, there is a clear correlation between digestible dietary TRP and animal behavior.

TRP’s influence on behavior

One notable study on the subject was conducted by researchers in the UK who developed swine diets with minor changes in TRP level (0.12, 0.22, or 0.32 percent). The study demonstrated clearly that small changes in TRP levels can have sweeping effects on swine behavior.

In the study, two levels of starch (35 or 45 percent) were also evaluated since carbohydrates may influence TRP uptake by the brain. The researchers used 21.9 kg pigs that were placed into groups of 10 and fed the treatment diets. Behavior was observed and recorded in daily sessions over the course of 11 weeks. The pigs were also subjected to a “model tail test,” which consisted of cords that had been soaked in either whole pig’s blood or water.

This experiment determined if the diet had any effect on tail biting, and was recorded as a blood preference score.

The researchers’ findings are shown in Figure 1. Pigs that were fed the lowest TRP level, with the lowest starch level (far-left bar within each group of bars), spent the least amount of time sleeping, most time “nosing” things and most time engaging in behaviors directed at other pigs. They also had the highest blood preference scores (tail biting indicator) and worst growth.

When pigs were fed only 0.1 percent more, TRP and starch levels remained unchanged (second from left bar within each group of bars), and the behavioral indicators and growth both improved. Moving to the next bar (third from left within each group of bars), TRP levels were maintained, while starch was increased, and some interesting observations were reported. The pigs with a diet higher in starch slept less, engaged in more “nosing” and pig-directed behaviors (including blood preference) and growth showed no changes — thus, increasing starch while keeping tryptophan constant, if anything, worsened behavior, but did not affect weight gain.

Finally, adding 0.1 percent more TRP, with starch constant (far right bars), improved pig behavior and did not affect weight gain.

Consider the big picture when formulating

Current formulation practices, including the use of lower-cost ingredients and reduced protein diets, have the potential to detrimentally affect pig behavior. This is shown in Figure 2.

Figure 2 demonstrates that as the use of dried distillers grains with solubles (DDGS) from the ethanol industry increases the amount of TRP in a typical corn, solvent-extracted soybean meal diet decreases. When dietary crude protein levels are reduced further, swine diets easily become TRP deficient. As this happens, the amount of starch is maintained, or increased. Further proving that current formulation practices have the combined effects of reducing TRP, while increasing starch, which is the worst possible scenario in terms of resulting behavior.

To add complexity, a new level of uncertainty emerges with the use of DDGS considering the high variability in protein levels common with DDGS.

A review of the literature provides more evidence that nutrition/diet formulation, growth performance, and animal behavior and welfare are clearly intertwined. For example, pigs that were fed “inadequate protein” diets had a clear preference for blood in a tail-biting simulation experiment. Also, group-housed sows fed a 20 percent DDGS diet fought with each other for longer periods of time and had higher levels of stress hormones; compared to sows fed DDGS-free diets.

Finally, piglets that engaged in more tail biting had lower body weights compared to those that tended to not bite tails. So, what can be done?

Strategies for ensuing proper inclusions

Feed formulators can take a few steps to better predict TRP’s role on swine behavior:

  1. Check the lab First, TRP measurement in feeds is more challenging than other amino acids. Therefore, it’s worth your time to speak with your lab. You can’t formulate properly with inaccurate data. If the lab can walk through their procedures, especially those with TRP, then you can be reasonably assured of reliable data. If your lab doesn’t understand the differences between TRP and other amino acids, then you should be very concerned about the results.
  2. Include L-TRP in the diet One obvious approach to low TRP levels is to add synthetic TRP to correct the imbalance created by DDGS. However, when adding synthetic amino acids great care must be taken in order to avoid creating another imbalance by over-supplementing and hindering performance. A little may be good, but a lot is usually not better, and can even be harmful.
  3. Use high-quality ingredients The negative effects of using one lower-quality ingredient can be offset by including a high-quality ingredient. For example, properly processed, extruded and expelled soy meal has higher-digestible amino acids, more residual oil; and less water than commodity, solvent-extracted soybean meal. As a result, swine diets with extruded, expelled soy meal will provide an increased level and balance of amino acids; as opposed to including individual, synthetic amino acids. Therefore, a more complete delivery of amino acids is achieved, which is beneficial for both growth performance and behavior.Also, the residual oil in the extruded, expelled soy meal will enhance digestion of amino acids in other ingredients, such as DDGS.   

The future is now

As nutrition, growth performance and animal welfare are intimately related, opportunities exist to formulate diets that enhance performance and well-being, and perhaps even to increase stocking density to allow for more throughput.

At a time when approaches from all areas of food production are questioned, nutritionists and pork producers can take simple steps to differentiate themselves from others. 

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