The Performance of Growing-Finishing Pigs Fed Diets with Reduced Crude Protein

By John F. Patience, A. Denise Beaulieu, Ruurd T. Zijlstra, Doug A. Gillis and James Usry, Prairie Swine Centre Inc. - Successful formulation of low protein diets increases our flexibility in formulating practical diets, provides us with another tool to lower nitrogen output in the slurry and to reduce greenhouse gas emissions. This experiment was conducted to compare the performance of pigs fed regular protein versus low protein diets.
calendar icon 30 November 2003
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An intermediate protein diet was also employed. Average daily gain, average daily feed intake and feed efficiency were unaffected by dietary treatment. Most carcass characteristics, including index, lean yield and backfat thickness were unaffected by treatment; however, loin muscle thickness was increased on the low protein diet. Lower crude protein diets can be fed successfully without negatively impacting performance or carcass quality.


There is increasing interest in formulating diets with lower crude protein content. The declining cost of synthetic amino acids, a desire to minimize the nitrogen content in the slurry and interest in reducing greenhouse gases all contribute to this interest. However, in the past, low crude protein diets sometimes reduced growth performance and often resulted in fatter carcasses.


The objective of this experiment was to determine if lower crude protein levels could be fed throughout the growout period, supporting maximum animal performance and maintaining carcass quality.


This experiment employed three dietary treatments (Table 1). The control diet was formulated to contain a level of crude protein that required no more than 0.1% L-Lysine HCl to meet the pig’s requirement for lysine, i.e. a typical diet used by the pork industry today. The low protein diet was formulated with the lowest possible crude protein level without using any synthetic L-tryptophan. In other words, levels of L-lysine HCl, DL-methionine and L-threonine were allowed to float in order to meet the pig’s requirement for these three essential amino acids.

However, synthetic L-tryptophan was not included in the formulation. This resulted in diets that contained as much as 3.5 kg L-lysine HCl, 1.4 kg l-threonine and 40 g dl-methionine per tonne of complete feed; these levels are clearly well above current commercial practice. A third diet was formulated to be intermediate in crude protein level between the other two. Diets were formulated to maintain a constant NE:Lysine ratio and equal levels of minerals and vitamins. Dietary electrolyte balance was similar across treatments.

There were a total of 5 pens and 110 pigs per treatment for a total of 660 pigs (330 gilts and 330 barrows). All pigs were housed in fully slatted concrete floored pens measuring 5.8 X 2.4 m. with spindle penning dividers. Pigs were housed 22 to a pen, providing 0.65 m2/pig. Pigs were on test from 30 kg to 115 kg.

Results and Discussion

Overall, performance was excellent, with growth rates averaging 959 g/d. Feed conversion was also a very good 0.359, or 2.79:1. The uniformity of performance was also very good, with the SEM for daily gain only 8 g/d and for feed intake only 25 g/d.

There were no significant effects of crude protein on average daily gain, average daily feed or feed efficiency (P>0.10). However, there was a significant interaction between treatment and days on test (P<0.05).

Reducing crude protein had no negative effects on carcass quality; surprisingly, the lowest crude protein diet resulted in the thickest loin (P<0.05). Premiums were higher on the low protein diet (Table 3) as was the returns over feed cost. The feed cost considers the cost of the diet and days on test, which increased as dietary crude protein decreased. As expected, gilts indexed higher than barrows (111.9 vs. 109.7), with higher lean yield (60.4% vs 59.2%), less backfat (19.1 mm vs 21.4 mm), a thicker loin (61.6 mm vs 59.0 mm), a wider backfat:loin spread(42.5 mm vs 37.6 mm) and earning higher quality premiums ($4.83 vs $4.07). These gender effects are all within the expected range. The thicker loin on the low protein diet was unexpected and needs to be repeated to see if this effect is real.

Detailed results of this experiment can be obtained by requesting Monograph No. 02-03 from the Prairie Swine Centre.


When diets are formulated on a net energy basis, synthetic amino aids used judiciously, and dietary electrolyte balance is maintained reasonably constant, crude protein levels can be reduced and performance maintained. Indeed, as evidenced by the thicker loin eye on the low protein diet, carcass quality may be improved.


Funding for this project from Ajinomoto Heartland Inc., Chicago, IL is gratefully acknowledged. Strategic program funding is provided to the Prairie Swine Centre by Sask Pork, Alberta Pork, the Manitoba Pork Council, and the Saskatchewan Agriculture Development Fund.

Source - Ruurd T. Zijlstra, Ph.D - Prairie Swine Centre - November 2003
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