Protein in pig diets: Meeting needs, avoiding excess

Protein is needed in pig feeds only because it supplies amino acids necessary to the animal. But of the total amino acids in the diet, only the digestible amino acids are biologically available to the animal.

Effect of excess protein in feeds

There are three primary reasons why we might limit the crude protein (CP) level in pig feeds:

1. Price.  Protein feeds usually cost more per tonne than cereals.
2. Pig performance.  Feed conversion efficiency (FCE) will be better on low protein diets provided amino acid requirements are met
3. Organic nitrogen excretion.  Because it is determined by CP in the diet, excess protein means more organic nitrogen excreted and possibly the need for more land to utilize this nitrogen.

While the pig needs the 10 essential amino acids in the diet, and in the correct proportions, we normally concern ourselves with five amino acids which are in relatively short supply in the common feed ingredients. These are lysine, methionine, methionine plus cystine, threonine and tryptophan. If the levels of all five in the diet are adequate and the total amount of digestible protein is adequate, then the levels of other essential amino acids will usually be sufficient.

A protein that contains the perfect balance of the individual essential amino acids combined with adequate amounts of non-essential amino acids is referred to as "ideal protein." The ideal blend is expressed relative to the lysine requirement, as shown in Table 1 for the most limiting amino acids in barley-wheat-soyabean meal diets.

The protein feeds most useful in pig feeds are those high in lysine, methionine and threonine. Ingredients vary widely in the amount of proteins they contain that are most valuable in pig diets. For example, lysine varies from about 8 percent of the protein in whey to 2.2 percent of the protein in sorghum. Pig feeds should have about 6 to 7 percent lysine in the protein. Methionine content of fish meals can be about 3 percent of the protein, while peas, beans and lupins are under 1 percent. Whey protein is high in threonine (6 to 7 percent of protein) while wheat is low (about 2.75 percent of the protein). This means that as we include "non-traditional" ingredients in pig feeds, we must do so with care and consider the content of several amino acids.

Ingredients vary

Digestibility of amino acids varies between ingredients and between amino acids within a single ingredient (Table 2). While using mainly cereal-based diets this was not a problem, but with current high cereal prices there will be pressure to use a wider range of ingredients and especially to use cereal by-products. We will now need to compare diets and express the needs of the pig on the content of digestible amino acids.

A shortage of the amino acid which is most limiting relative to the requirement will limit pig performance, as shown in Table 3. This could result if synthetic lysine is supplemented without adjustment of the other amino acids. On very low protein diets supplemented with lysine, threonine and methionine, tryptophan deficiency might be the reason. Synthetic lysine can be poorly utilized if included at a high level and the pigs are fed only once daily or in liquid feed where fermentation occurs.

Too much of any one amino acid may depress pig performance, too, as shown by Karen O'Connell at the 2004 Teagasc pig conference where high levels of lysine resulted in reduced growth rate and poorer FCE.

Table 4 shows two finisher diets, one high protein and one low protein, formulated to contain a minimum of 11g/kg lysine, 3.3g/kg methionine, 6.6g/kg methionine plus cystine, 7.2g/kg threonine and 2.0 g/kg tryptophan. Both have the same levels of the critical amino acids.

Since both diets are considered equally capable of meeting the needs of the pig, they should both support at least equal growth rate. However, there is less excess N to be excreted from the low protein diet and therefore FCE will be better.

There is abundant other evidence that low protein diets support better FCE, intake and growth rate provided the amino acid needs of the pig are met (e.g. Table 5 from University College Dublin). As a rule of thumb, each 1 percent extra protein requires 1 percent of the feed energy for conversion to urea in the body and excretion. On that basis, the low protein diet in Table 3 will support 5 percent better FCE making it worth 5 percent more per tonne or 11 at present prices but the low protein diet will be dearer.

Using low protein diets in all stages of production, it would be possible to reduce the ON excretion from each sow to below 70kg as illustrated in Table 6.

Protein and ON excretion

The amount of nitrogen in the manure is the difference between the input (amount of feed and its crude protein content) and the output in pig meat. We assume that the loss of N to the atmosphere as NH3 and N2 is a constant percentage of that excreted.

A larger number of pigs sold, heavier slaughter weight and poorer FCE will all inflate the ON output per sow. Based on analysis of diets from feed mills DAFF in 2006 increased their estimate of ON excretion from 75kg/sow per year to 87kg. This resulted in a 12 percent increase in the amount of land required. This makes compliance with the SI 378 of 2006 more difficult. Protein levels in feeds in 2007 are marginally lower than in 2006 but could be reduced substantially.

Low protein diets will also result in:

• less ammonia in the atmosphere
• less odour from the manure
• less water consumed leading to a lower volume of manure

We have developed current recommendations for energy and amino acid content of feeds for different categories of pigs (see www.feedindustrynetwork.com for a table of these recommendations). Adjustments are needed as dietary energy changes.

Lower protein levels in pig feeds are desirable from the point of view of pig performance and compliance with legislation. Therefore, feed manufacturers need to reassess protein levels in feeds. Having the correct balance of digestible amino acids is critical.