Lysine availability in cottonseed meal limits its value in pig diets

Why cottonseed meal needed a second look#

Cottonseed meal has long served as a protein concentrate for growing pigs because it is reasonably priced and widely available as a by-product of cotton processing. Its protein content ranges from 32 to 45%, which sounds competitive with other concentrates. The catch has always been gossypol, a pigment in the seed that protects the plant from insects but is toxic to pigs if present in free, unbound form.

For decades the assumption was that gossypol toxicity set the ceiling on how much cottonseed meal could go into a pig diet. The paper by Batterham challenges that assumption directly. Recent work cited in the paper indicated that the amino acid quality of the meal, rather than its residual free gossypol content, was the real constraint on its feeding value. That shift in thinking mattered because it pointed nutritionists and processors toward a different problem to solve, one buried in how the meal is made rather than in how much of it is fed.

Working out which factor actually limited performance required separating the effects of gossypol from the effects of processing on the protein itself, which is what the bulk of the research described in the paper set out to do.

Testing the limits of gossypol tolerance in pig diets#

Gossypol in cottonseed meal exists in two states: bound, where heat applied during processing locks it away, and free, where it remains chemically reactive and can harm the animal. The paper reports that locally produced cottonseed meals typically carry 0.03 to 0.09% free gossypol. Ferrous sulphate can be added to diets to bind this free gossypol in a 1:1 weight ratio with iron, inactivating it.

Citing Tanksley and Knabe (1981), the paper notes a dietary free gossypol threshold of 100 ppm below which growing pigs show no adverse reaction, rising to 500 ppm once ferrous sulphate is included in the ration. Working through those tolerances against a typical free gossypol content of 0.05% in the meal, the paper concludes that a considerable proportion of the diet could be made up of cottonseed meal without iron supplementation, and an unlimited amount with it.

That conclusion effectively ruled gossypol toxicity out as the main practical barrier to using cottonseed meal in pig diets, at least at the inclusion rates likely to be considered on-farm. It also set up the central question the rest of the research pursued: if gossypol was not the limiting factor, what was?

Tracking the damage to lysine during processing#

Cottonseed is conditioned, flaked, and heated with moisture to begin fixing gossypol and loosen the oil for extraction, then run through mechanical presses to squeeze out part of that oil before a solvent step pulls out what remains, and finally reheated to drive off the solvent and complete the binding of free gossypol. Batterham's work, conducted with a commercial cottonseed meal processing plant, aimed to pinpoint where in this sequence lysine was being damaged.

An initial experiment compared three products: white meal, taken straight after solvent extraction; a second-deck sample representing minimal post-extraction drying; and normally processed cottonseed meal. All three were included at approximately 20% in wheat-based rations deliberately short of lysine, given at a controlled daily amount to pigs growing from 20 to 45 kg liveweight, with ferrous sulphate added to control gossypol effects. A wheat-soyabean meal diet matched for lysine content served as the control.

There was no difference in performance among pigs fed the three cottonseed meal samples, but all were inferior to the soyabean meal diet. This indicated that the damage to lysine had already occurred before solvent extraction, that pigs tolerated the higher free gossypol level in the white meal diet once iron sulphate was added, and that cottonseed meal remained a poorer lysine source than soyabean meal regardless of the later drying steps.

Trying to rescue lysine availability in cottonseed meal by changing processing#

Having traced the damage to the pre-solvent-extraction stage, the researchers then tried to alter conditions ahead of expeller extraction to see whether lysine availability could be protected. The attempts ran into practical limits of the processing equipment itself: reducing moisture content produced a dry, powdery meal that would not move properly through a system built for moister material, while keeping moisture high and lowering temperature produced material that was too wet to flow at all.

Within those constraints, the researchers managed only to shift free gossypol content between 0.06% in the normal meal and 0.086% in an experimental meal. A slope-ratio assay with growing pigs was then used to measure lysine availability in both meals alongside soyabean meal. The results were telling: 0.27 for the normal cottonseed meal, 0.30 for the experimental meal, and 0.90 for soyabean meal.

Those figures confirmed that cottonseed meal lysine availability sat well below that of soyabean meal, and that the modest adjustments possible within the existing plant made little practical difference. The paper's own interpretation is that existing Australian processing technology offers little scope for improvement, and that meaningful gains would require substantial design changes or different processing systems altogether.

Checking whether other amino acids are affected too#

Lysine is a dibasic amino acid prone to Maillard-type reactions with reducing sugars during heating, which raised the question of whether other essential amino acids in cottonseed meal might be comparatively unaffected. If so, simply adding free lysine to diets could restore much of the meal's protein quality. Testing the availability of every essential amino acid through slope-ratio assays was considered too costly and time-consuming, so the researchers turned to ileal digestibility, measured at the end of the small intestine, as a potentially faster proxy, an approach earlier suggested by Leibholz (1985a, 1985b), whose work had found that lysine and methionine measured this way in young pigs closely matched what the animal actually absorbed.

Apparent ileal digestibility of amino acids was measured in a prepress solvent-extracted cottonseed meal and compared with soyabean meal as a control. Most amino acids showed only slightly lower digestibility in cottonseed meal than in soyabean meal, consistent with earlier literature. Tryptophan stood out as considerably lower in cottonseed meal.

The more important test, though, was whether ileal digestibility actually predicted how much of an amino acid the pig could use for growth, which is a different question from how much simply disappears from the gut.

Why ileal digestibility overstated true lysine availability#

To test this, sugar-based diets were formulated using cottonseed meal or soyabean meal as the sole lysine source, set to 0.36 g ileal digestible lysine per MJ DE, with other amino acids added so lysine remained limiting. The pigs again grew from 20 to 45 kg on a controlled daily allowance, and their lysine retention was measured.

Growth rate and feed conversion were poorer on cottonseed meal than soyabean meal. Only 28% of estimated total lysine and 38% of estimated ileal digestible lysine intake ended up retained in the carcasses of pigs fed cottonseed meal, compared with 65% and 73% respectively for pigs fed soyabean meal. Using availability values of 0.4 for cottonseed meal and 0.89 for soyabean meal from the Standing Committee on Agriculture (1987), estimated retention of available lysine came out closer between the two diets, though the paper records this as 0.73 for soyabean meal.

This told the researchers that ileal digestibility was not a safe stand-in for true lysine availability, because a meaningful share of the ileal digestible lysine in cottonseed meal was apparently absorbed in a form the pig could not use. It also showed that even on the soyabean meal diet, only around 70% of available lysine intake was retained, implying that roughly 30% goes to other functions or is lost through protein turnover.

Threonine, methionine, and the overall pattern of damage#

Similar experiments were run for threonine and methionine. For threonine, conducted by Beech and colleagues, growth rate and feed conversion were again poorer on cottonseed meal, with 45% of total threonine and 58% of ileal digestible threonine retained, against 64% and 76% for soyabean meal. Assuming a threonine availability of 0.85 in soyabean meal, close to its ileal digestibility value, the paper estimates cottonseed meal threonine availability at about 0.6, suggesting threonine suffers less processing damage than lysine.

For methionine, only growth data were available at the time of writing, but the same pattern held: poorer growth and feed conversion on cottonseed meal, with smaller performance gaps than seen for lysine, again pointing to comparatively less damage to methionine during processing.

Taken together, these amino acid results suggest that processing damage is not uniform across the protein, with lysine the most severely affected. This matters for diet formulation because it means the gap cannot be closed simply by topping up with free lysine. We think the practical weight of this evidence sits more with the direction of the findings than with the precise retention figures, given the preliminary nature of the threonine and methionine data and the narrow range of processing conditions achievable in the plant trial, but the overall conclusion that formulation needs to move to an available-amino-acid basis looks well supported by the lysine and threonine data together.

What this means for formulating diets with cottonseed meal#

The practical message from the paper is that diets built around cottonseed meal should be formulated on an available amino acid basis rather than on total or ileal digestible amino acid content, because both of the latter measures overstate what the pig can actually use. Gossypol toxicity, by contrast, appears manageable with ferrous sulphate supplementation at the inclusion rates likely to be used.

The author's own assessment is that current Australian processing technology leaves little room to recover lost lysine availability, and that new processing systems designed specifically to protect amino acids while still extracting oil efficiently would be needed before real gains could be made. That is a structural limitation rather than one that can be solved through diet formulation alone.

Batterham's broader body of work on protein and amino acid handling in pig diets extends this theme, including the related study on nutritional implications of heating proteins, part of the same RAAN Conference Proceedings collection within the Livestock Library research index.

Sources and further reading#

• Trove library search: find a library that holds the paper
• Feedipedia animal feed database: an open-access database of animal feeds
• Australian Meat Processor Corporation: research for red meat processors

Questions#

— The Livestock Library team