Meeting nutrient requirements of growing pigs through daily targets, not fixed tables

Why fixed feeding tables were causing confusion#

The paper opens by noting that the Agricultural Research Council and the National Research Council had each published their own estimates of pig nutrient requirements, in 1967, 1968 and 1973, and that the English and American figures differed markedly from one another. Swainston treats this gap as a sign that something was wrong with the whole approach, not just with the numbers themselves.

The argument runs that nutritionists had spent a great deal of effort chasing a perfect feed formulation and a perfect amino acid balance, without properly allowing for the fact that daily feed intake, growth potential, nutrient availability, market requirements, environment and disease all vary enormously between pigs and between piggeries. Any single recommended ration was always going to suit some animals while under- or over-feeding others.

Swainston points to work by Whittemore and Fawcett and by Whittemore and Elsley, along with a study by Thorbek, as offering a different way forward. Their approach set requirements against specific daily targets for fat-free growth and fat deposition, rather than a blanket figure for an average pig. Whittemore and Fawcett had gone further and built a computer model of the growing pig from these physiological relationships, a model whose predictions reportedly matched results from live pig experiments closely enough to support its validity.

The remainder of the paper works through what that daily-target approach means in practice for protein, energy, vitamins, minerals and digestibility, drawing throughout on the structure of the Whittemore and Fawcett model.

How the model frames nutrient requirements of growing pigs#

The model allows for differences in the pig's maximum potential daily protein deposition, its daily feed intake, the specification of its diet, its starting and slaughter weight or age, the temperature of its environment, and carcase quality as measured by backfat thickness at the P2 site. Feeding this detail in, pig by pig, is what lets the simulation track growth on a daily basis rather than averaging across a whole batch.

Underpinning this is a view of growth priorities: skeletal development comes first, then muscle, and only once those needs are met does surplus energy get stored as fat. Up to a growth rate of 550 g per day, the paper describes liveweight gain as splitting evenly between protein and fat by weight, so each 100 g of gain includes 20 g of fat and 80 g of lean, with that lean containing 20 g of protein.

Young pigs often cannot eat enough feed to meet their potential before they reach 30 to 40 kg liveweight, and this limit on intake, rather than any shortfall in the diet's balance, is given as the main reason lean growth falls short of potential in younger animals. Backfat thickness at P2 is used in the paper as a practical way of inferring the proportion of fat in the carcase, and from there, the rate of protein deposition, for pigs growing at rates of 500 g per day and 650 g per day from 20 kg liveweight.

Setting protein and amino acid targets#

Protein requirement is broken into three parts: a maintenance component tied to lean body mass, which the paper scales down from roughly one eighth of one percent of body weight in small pigs to around one twentieth of one percent in large pigs; the protein cost of lean growth itself, given as about 22% protein by weight; and an adjustment for the biological value and digestibility of the dietary protein actually fed.

On amino acids, the paper sets out recommended levels alongside figures extrapolated from the AEC for comparison, and flags several complications that research had only recently uncovered. Batterham and O'Neil had shown that the proportion of synthetic lysine a pig can actually use falls when it is fed in one daily meal rather than split across six feeds spaced three hours apart, with roughly two-thirds availability reported under the once-daily regime. Diets lower in protein but topped up heavily with synthetic lysine could still support strong growth rates and leaner carcases, the paper notes, yet protein deposition and feed conversion were not fully restored under those regimes, work attributed to the AEC and to Campbell. Lysine requirement itself was found to climb as surplus crude protein increased, by 0.29 for each 10 g of surplus crude protein present.

Energy needs, temperature and the hidden cost of protein deposition#

Maintenance energy requirement is put at a range running from 0.4 up to 0.5 MJ of metabolisable energy per kilogram of metabolic weight, following Whittemore and Elsley. Beyond maintenance, energy is needed both for the protein and fat actually laid down and for driving the biochemical processes of growth, much of which is lost as heat.

A notable point in the paper is that laying down protein costs more energy gram for gram than laying down fat, yet because lean tissue holds far less energy per unit weight than fatty tissue, growing lean is still the cheaper option once the whole tissue is considered. The energy cost of protein growth is put at between 45 and 80 MJ per kilogram of protein, a wide range explained by the fact that body protein is constantly broken down and rebuilt in proportion to lean mass, with around 7.5 MJ of metabolisable energy needed per kilogram of new protein just to rearrange amino acids.

Cold environments add a further cost. Below a pig's lower critical temperature, the point at which no production losses occur, the paper states that an extra 0.016 MJ of metabolisable energy per kilogram of metabolic body weight is needed for every degree Celsius below that threshold, and that pigs growing more slowly than their potential need a warmer environment before losses start, since they generate less metabolic heat of their own.

Vitamins, minerals and water#

The paper compares ARC and NRC estimates of total vitamin and mineral requirements against levels the AEC recommended adding per kilogram of feed, and adds some practical notes from more recent findings. Pigs were thought to need about 15 i.u. of vitamin E per kilogram of feed, and field reports had linked biotin deficiency to cracked heels, mild dermatitis and hair loss in pigs kept confined indoors and fed cereal and meat meal based rations.

On ascorbic acid, the paper is cautious: most pigs thrive without a dietary source because they can make their own, though severely stressed piglets eating very little feed may benefit from a supplement because their energy intake is otherwise so low.

For calcium and phosphorus, the lower figures from the NRC were considered appropriate mainly for corn-soyabean diets low in phytate phosphorus and supplemented with available phosphorus; the paper suggests keeping the calcium to phosphorus ratio between 1.2 and 1.4 to one on a long-term basis. Zinc requirements rise in the presence of high calcium, phytic acid and copper, with supplements of 100 mg per kilogram of feed commonly used. Molybdenum is included because of the scabby hip syndrome reported in broilers in Australia, with a tentative supplement range of 0.2 to 2 mg per kg suggested, alongside a note that pigs can tolerate molybdenum levels far higher than ruminants can safely handle. Water is recommended at a minimum of two litres per kilogram of feed, provided twice daily where supplies are scarce.

Digestibility and processing change what a feed actually delivers#

A major theme of the paper is that the nutrient value of an ingredient is not a fixed number on a feed table; it depends on how the ingredient was processed and on what else is in the diet. Digestibility of energy in grain is given as about 60% for whole grain, rising through 73% for broken grain and 80% for coarsely ground grain to 85% for finely ground grain, though the paper also notes wheat can become less digestible when ground too finely because it forms an indigestible dough.

Protein digestion is described as best measured at the terminal ileum, because protein reaching the large intestine is fermented by micro-organisms and can no longer contribute amino acids to the pig, even though that fermentation yields volatile fatty acids that do supply some energy. Raw soyabeans and several other ingredients, including other beans, rye, barley, wheat, triticale, lucerne and potatoes, were all found to contain compounds that can inhibit protein digestibility, effects that suitable cooking can prevent. Compounds in cottonseed meal and in sorghum were noted for binding protein and lowering amino acid availability, while excessive heating of meat meal in particular was flagged as a common cause of reduced digestibility.

Pelleting was reported to improve pig performance, with the strongest responses seen in rations built around barley or containing pollard and bran, and weaker responses in corn-based rations. Saunders, Walker and Kohler, working with chickens, linked the benefit to pelleting rupturing aleurone cells in the feed, a physical effect of the die and steam conditioning rather than a chemical one, since autoclaving bran without pelleting reduced digestibility instead.

Stress, disease and drugs can swamp any nutrient calculation#

The paper is candid that few studies had actually measured how stress and disease change nutrient requirements, despite how costly these factors clearly are in practice. Enzootic pneumonia and mange were described as insidious diseases that reduce growth rate, raise metabolic rate and cut feed intake under group housing, with feed conversion deteriorations in the region of 8015% quoted for each, read in the paper as a range spanning roughly 8 to 15%.

Social stress between pen mates was identified as a major source of the variation seen between individual pigs, and housing littermates together from birth right through to slaughter was said to let them reach bacon weight two weeks sooner than pigs reared under more usual mixing and regrouping. In a striking illustration of how stress alone can distort results, Ostrowski is reported to have deliberately upset a pig confined in a metabolism cage, which was enough to drop its feed digestibility from 75% down to 50%.

Feed medications were found to lift growth rate and feed conversion by around ten percent in younger pigs weighing up to roughly 20 to 30 kg, with much smaller but still potentially profitable responses in larger pigs, though the mode of action of many of these drugs remained uncertain and responses varied considerably between piggeries and over time. Given how large these effects can be, Swainston leaves open the question of how a nutritionist should adjust a ration to allow for stress, disease and medication at the same time as meeting a pig's baseline nutrient requirements of growing pigs. Read alongside other work gathered in the RAAN Conference Proceedings collection and the wider Recent Advances in Animal Nutrition community, this uncertainty looks like one of the clearer unfinished threads in Australian pig nutrition research of the period, and we think it deserves as much weight as the better-known arguments about amino acid ratios. Readers comparing how other livestock papers have framed the word meeting in their own titles may also find it useful to look at the record for meeting water requirements of cattle, a separate study on a different species and resource altogether.

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
• Poultry Hub Australia: poultry science information
• AgriFutures Australia: which funds chicken meat and other rural industry research

Questions#

— The Livestock Library team