Energy is a big challenge when evaluating ingredients, formulating diets
Level in the diet can impact return over feed cost by as much as $3 per pig sold.
By John Patience
Everyone recognizes that energy is the most expensive component of the pig’s diet. It might also be the most complicated. I have been studying energy for the last 20 or so years, and in that time, I have gained a healthy respect for the subject. Some days, just thinking about energy makes my head hurt. Other days, I am in awe of everything that has to do with energy – how energy from the sun is converted into energy that the pig (and us humans for that matter) can use in its diet, how we supply energy to the pig and how the pig uses that energy to keep itself alive, to grow, to produce little piglets and to produce milk to nourish those piglets.
Because energy represents such a major portion of the cost of a pig’s diet, it is increasingly accepted that success in pork production will depend, in part, on who can buy or produce dietary calories the cheapest and who can convert those calories most efficiently into meat protein. To use energy values in diet formulation, we need to understand where they came from, their accuracy, and their limitations.
However, the very elegance of energy utilization and conversion into animal protein is also the reason for the complexity of the subject. First, energy comes to the pig from four different sources – protein, starch, fat and fiber - and each of these is used differently by the pig. Some of the energy such as starch can be used almost “as is;” it requires little more than digestion to be able to enter the metabolic machinery of the pig. Other sources of energy such as protein and fiber have to undergo considerable modification before the pig can use them. This makes them less efficient sources of energy.
To complicate things further, the efficiency with which the pig uses each energy source will depend on what the energy is being used for. For example, the efficiency with which fat is used can vary from 66% to 90%. The variation in starch efficiency is much less, in the range of 68% to 74%.
These are significant differences, and help to explain the difficulty we sometimes have in predicting the pig’s response to changes in the energy content of a diet, especially when we use diverse ingredients.
Figure 1 shows the practical implications of this topic. Increasing the quantity of corn distillers dried grains from 0% to 45% increases the amount of energy coming from fiber by 43%, from fat by 50%, and from protein by 14%. The quantity of energy coming from starch declines by 16% because there is very, very little starch in DDGS. If we formulate a diet by simply looking at the total energy in the diet, this change in the source of energy will be missed.
There are other issues as well. When the energy content of corn is determined, the most common method is to conduct what is called a balance study, where pigs are fed the different corn samples and feces are collected.
The difference between the energy consumed by the pig and excreted in the feces is called digestibility and this is used to calculate the digestible energy of corn; when the quantity of energy excreted in the urine is also subtracted, this is called metabolizable energy.
It is assumed that differences in digestible and metabolizable energy are due to digestibility. However, as shown in Figure 2, in this example, the differences between high and low energy corn is clearly not due to digestibility, because it is the same for all samples; the difference is due to fermentability in the large intestine and cecum of the pigs.
This is significant because the energy obtained by the pig from fermenting ingredients is used less efficiently than energy obtained by digestion. Fortunately, the energy in corn does not vary very much, so this issue may not be very important in the grand scheme of things. However, everything we do in the field of nutrition is to control the variability that we can; in this way, we come closer to achieving our overarching goal of formulating diets with predictable performance and financial outcomes.
As mentioned above, the digestibility of energy is based on the difference between the energy that is consumed by the pig and what appears in the feces; this is called apparent total tract digestibility or ATTD. This assumes that all of the energy in the feces is derived from the diet. It is common practice when measuring the digestibility of amino acids to acknowledge that not all of the amino acids come from the diet; some come from the intestine itself, in which is called endogenous secretions – enzymes, sloughed cells and so on.
The same appears to be true with fat, as shown in Figure 3; if fat digestibility is adjusted for endogenous secretions, in what is called true total tract digestibility or TTTD, fat digestibility is shown to be – correctly – considerably higher. By ignoring this, we are underestimating the energy derived from fat in the diet, and the magnitude of this error is greatest when the level of fat in the diet is low – the exact situation we are facing now when adding fat to the diet is very expensive.
Finally, nutritionists are faced with the decision on which energy system to utilize in their formulation programs. It is not an easy decision because there are many factors to take into consideration and there are many options.
In North America, many nutritionists have converted from the metabolizable energy (ME) system to the net energy system (NE), but many have chosen not to.
I personally have a preference for net energy because it gets us a bit closer to measuring the actual amount of energy available to the pig for productive purposes. The benefit is not huge, but it is measurable and repeatable.
Net energy has the advantage of considering what is called heat increment – the quantity of energy that is required to ingest and digest feed, to convert metabolizable energy so it can be used by the pig and to support normal physical activity.
Thus, the energy included in heat increment is not available for productive purposes; it may provide value to the pig housed in cooler climates but it will be a decided negative when pigs are housed in hot climates (this is why we prefer to feed lower protein and lower fiber diets in the warm summer months).
Heat increment averaged across many common ingredients is about 22% of gross energy, which is only slightly less than the average energy lost in the feces, which is about 26%. However, while the means are similar, there is a much bigger range in fecal losses of energy than that lost as heat increment. Nonetheless, it is a significant factor.
For example, about 19% of gross energy in corn is lost as heat increment – one of the lower ingredients in this respect - while about 28% is lost in soybean meal (although recent research suggests that this amount might be over-estimated). More to the point, net energy tends to predict pig performance slightly better than metabolizable energy.
As shown in Figure 4, the measured quantity of energy utilized per lb. of carcass weight is much more consistent across different types of diet with NE than with ME, which is what one would expect. The control was primarily corn-based, while the “D” diets contained DDGS and the “DC” diets contain DDGS and corn germ meal.
This article has discussed some of the challenges nutritionists face when evaluating ingredients and when formulating energy levels in pig diets. Choosing the right – or wrong – energy level in the diet can impact return over feed cost by as much as $3 per pig sold. Not surprisingly, the difference is greatest when feed costs are higher.
Interestingly, the differences are similar whether the pigs are following a fast growth or slower growth curve.
This all adds up to the need to understand energy values, how they are determined and their innate flaws and limitations. The importance of having a trained nutritionist formulating diets becomes evident when one considers the impact of errors or the benefits of “getting it right.”
Quite simply, energy is a tough nut to crack and dealing with energy is where nutritionists really earn their pay.
Patience is a professor emeritus at Iowa State University
