It is generally assumed that feeding a higher nutrient density diet will enhance pig performance. The only outstanding question in most people’s minds is at what point does the higher cost of the high energy diet exceed the value of improved animal performance.

Confounding this logic is recent research at the Prairie Swine Centre showing that pigs do not always respond to higher energy diets with improved performance. Indeed, we have completed no less than 4 studies with nursery pigs showing no increase in growth rate when dietary energy was increased. Have we been wrong all these years in feeding high energy diets in order to achieve improved performance?

There are other reasons for wanting a better understanding of how the pig uses energy. For example, our knowledge of amino acid metabolism is rapidly increasing, with literally dozens of experiments on this subject completed each year. We are rapidly getting to the point where a nutritionist can estimate with a reasonable degree of accuracy, the optimum level of lysine and other amino acids required for a given farm operating under a given financial environment. However, before we can take full advantage of our knowledge on amino acids, we must have an equivalent understanding of energy – and that is certaintly not the case at the present time.

Clearly, there is a compelling need for much more information on the response of the pig to dietary energy concentration, whether they are in the nursery or in the growout barn. Not surprisingly, then, this has become a primary focus of our current research program. We are directing particular attention to the growing and finishing pig, since this is where the bulk of feed expenditures occur.

In our most recent experiment, we put a total of 300 pigs – 150 barrows and 150 gilts - on test from 31 kg to market at 115 kg. The experiment was carried out in three phases: 25 to 50, 50 to 80 and 80 to market. Five energy levels were compared, ranging from a low of 3,090 kcal DE/kg to a high of 3,570 kcal DE/kg; these levels were selected to represent the range in DE that might reasonably be utilized in a western Canadian context. The lysine:DE ratio was held constant, to ensure that amino acid levels did not limit the ability of the pigs to respond to dietary energy concentration. Diets were based on barley, wheat, soybean meal, canola meal and canola oil and were fed as a mash. The specifications of the diets are presented in Table 1.

We were very surprised to observe in this experiment that the pigs grew at the same rate per day, irrespective of dietary energy concentration. Average daily gain averaged 1.02 kg/day across all diets, and there were so statistically significant differences due to diet (Table 2). Barrows grew about 80 g/d faster than gilts, and the heavier pigs at the start of the trial grew 50 g/d faster than the lighter pigs.

As expected, feed conversion improved with increasing energy concentration, such that a 15.5% increase in diet DE, from the lowest to the highest energy diet, resulted in a 16.7% improvement in feed efficiency (Table 2). The improvement in feed efficiency confirms that the pigs were utilizing the additional energy present in the higher energy diets. We also determined the actual DE as compared to the formulated DE, and found them to be in close agreement (Table 1). We can therefore conclude that the absence of a growth response was not due to errors in formulating or manufacturing the diets.

The gilts had only a slight advantage over barrows, with respect to feed conversion and this difference was statistically significant. There was no significant difference in the feed conversion of the pigs that starter the experiment in the light group as compared to the heavy group.

No study on dietary energy would be complete without carcass information. Increasing dietary energy had no effect on loin thickness, but it did result in increased back fat and decreased lean yield (Table 3). The difference in backfat, in the order of 2.6 mm, was much larger than expected.

Not unexpectedly, barrows were fatter than gilts, by 4.4 mm and had a lower index, by 3.3 units. There were no differences between the light and heavy groups, which would be expected, since they were both marketed at the same weight. We also looked at the effect of dietary energy on the variability in performance and saw no effect. This is not the first time that we have observed no effect of feeding higher quality diets on variation.

Perhaps the most critical results are the economical analysis. Increasing dietary energy concentration increased feed costs by $11.75 per pig, from $37.76 to $49.52. Considering the revenues generated, the return over feed cost differed by $10.37 per pig sold across the range of diet DE levels.

The results of this experiment agree with earlier studies conducted at the Prairie Swine Centre in the nursery pig. However, they are quite contrary to what the industry expects to happen when higher energy diets are fed in the growout period. Certainly, we would not recommend that pork producers change their feeding program on the basis of a single experiment. However, given the very large differences in profitability observed in this experiment, we would strongly urge producers to re-evaluate their existing programs and perhaps run their own simple study to determine if they are feeding the optimum energy levels on their farm.

This experiment was conducted in a barn where feed intake is quite high. Because the results were unexpected, and because so many dollars are resting on the correct selection of dietary energy, we are going to repeat this experiment in another commercial barn, to see if these results can be replicated. We should have the results of that study next spring.