Feeding the young pig both adequately and economically has never been an easy task, but the manufacturer of piglet diets is now being asked to take further factors into consideration. Among these are the birth origins of the animal and even the possibility that part of its feed supply should be for reasons other than body maintenance and growth.
This second aspect was raised in 2010 at the world congress of the International Pig Veterinary Society, when Dr Douglas Burrin of the US Department of Agriculture’s Agricultural Research Service presented a lecture on the role of nutrition and intestinal adaptation in the weaned pig. In his remarks, he introduced the rather novel concept of also formulating to meet gut nutrient requirements.
Fuels for gut function
“Historically, weanling pig diets have been formulated largely to overcome the limitations or immaturity in digestive function in order to maximise growth of the whole animal,” he said. “However, with a new understanding of intestinal nutrient utilisation, it is now possible to formulate such diets with the specific goal of optimising the growth, function and health of the gut.”
Although most swine nutritionists have an extensive knowledge of the pig’s nutritional needs for growth, he told the meeting, research in recent years has extended the whole area of defining requirements so it also now includes nutrients needed specifically for the gut.
“Technical advances have enabled direct estimates of gut nutrient utilisation and the impact on whole animal nutrient metabolism,“ said Dr Burrin. “A major concept that has emerged from studies with young pigs is that non-essential amino acids are major gut fuels.”
The research into intestinal nutrient utilisation has suggested that some of the most promising candidates for manipulation in a new-look formulation strategy are the amino acids glutamine, glutamate and threonine with aspartate. From in-vivo studies in pigs, roughly 70-80% of the glutamate, glutamine and aspartate provided by the animal’s diet is taken up by the gut at the first pass and metabolised to carbon dioxide. “Glucose is also quantitatively an important oxidative fuel for the pig intestine. In absolute amounts, the intestinal utilisation of glucose is similar to the combined total from glutamate, glutamine and aspartate. However, the proportional use is lower, such that only about 20-30% of the dietary glucose is metabolised by the gut.”
The implications of the research conducted to date seem to be greatest with regard to the supply and use of amino acids in young pigs, to judge from Dr Burrin’s remarks. He described studies with piglets showing that the neonatal intestine played a key role in the metabolism of amino acids involved in the urea cycle, particularly arginine, proline, and ornithine.
There is extensive inter-conversion between these amino acids, he reported. The intestine represents an important site of net arginine synthesis in neonatal pigs. Moreover, the intestinal studies in pigs have demonstrated the extensive metabolism of other essential amino acids, including threonine, lysine, phenylalanine, branched-chain amino acids and methionine.
“It is generally considered that the primary metabolic fate of essential amino acids taken up by the gut is incorporation into tissue proteins. However, studies show that extensive irreversible catabolism and oxidation of amino acids occurs in the gut. Many of these essential amino acids are metabolised to other intermediates involved in intestinal function.
“For example, threonine is believed to be channeled into mucin synthesis and secreted by goblet cells, because mucin peptides are rich in threonine. Methionine may be converted to cysteine or s-adenosyl-methionine used in polyamine synthesis. Cysteine is used as a precursor for glutathione synthesis and maintenance of mucosal anti-oxidant status.”
The oxidation of some essential amino acids, such as the branched-chains and lysine, in the gut may be nutritionally significant, Dr Burrin continued. The high essential amino acid utilisation rate by gut tissues can impact significantly on the systemic availability of amino acids for lean tissue growth.
Current ideas from Europe and North America on the piglet’s need for dietary amino acids were summarised at the end of last year in a presentation to the Society of Feed Technologists, SFT, in the UK by Lars Sangill Andersen, feed application manager for Hamlet Protein A/S in Denmark. Amino acid values to supply within a piglet diet tend to be defined according to their relationship to lysine in an ideal protein combination, he pointed out.
Typically, this could mean methionine at 33% relative to lysine or methionine + cystine at 57%. Threonine could be at 65%, tryptophan at 20%, valine at 70%, isoleucine at 58% and histidine at 33%.
Modern formulations in practice consider digestible amino acids per kilogram of feed according to the energy level of the diet and the weight of the piglet. But as Mr Sangill Andersen pointed out, not all requirements have been defined even today and particularly not those for the youngest pigs. The nutritionist therefore must still work mainly from values found for larger growing pigs, despite the certainty that the needs of small piglets will be lower.
From his SFT presentation, accompanying Table 1 sets out some standardised ileal digestible amino acid values at different piglet weights and levels of dietary metabolisable energy. They are based on data established in France, Germany and the US.
As he remarked, however, issues on how to formulate effective feeds for young pigs go beyond the specification of amino acids or other individual nutrients. Among the questions for formulators to answer is whether the right ingredients have been chosen and whether the concentration of crude protein in the early diet might lead to problems of piglet diarrhoea.
It is possible to identify certain ‘good’ ingredients in terms of their suitability for piglet starter feeds, he declared, and also some ingredients that can be rated as ‘bad’ in this context. His own list under a classification of good included milk products such as skim milk power, lactose, whey and whey protein concentrate as well as spray-dried blood plasma, fishmeal, cooked cereals and protein concentrates processed from soybeans and potatoes.
Among ‘bad” ingredients, he indicated, would be soybean meal and raw (uncooked) cereals together with meals from other oilseeds such as rape and sunflower and also full-fat soy, in addition to red blood cells.
These ratings are based on a combination of experience and measurables, Lars Sangill Andersen added. We can measure trypsin-inhibitor levels, for example, besides knowing where a particular ingredient has been ineffectual. But the central issue in each case is one of digestibility.
The first requirement in feeding the pig after weaning is to provide a highly digestible diet because of the animal’s still-immature digestive system. When protein is highly digestible, its amino acids are readily available to the pig for use in healthy growth. By contrast, a protein source with poor digestibility not only wastes an expensive input, it also consumes extra energy in its disposal and it sends undigested amino acids into the colon where undesirable gut flora may develop so that the piglet suffers from diarrhoea.
Litter size effect
Diet selection for young pigs before weaning must also address changing circumstances brought by extra farm productivity, according to another Danish speaker to the SFT meeting. Dr Flemming Thorup, pig researcher with Denmark’s Agriculture & Food Council, drew attention to data from breeding units showing that modern sows are giving birth to more piglets.
As an example, a research report from Denmark at the start of 2011 commented that the number of pigs born nationally in each farrowing had increased steadily for 15 years and was now at an average of 16.1 pigs per litter.
Although this may be a good basis for greater efficiency in pork production, the report commented, larger litters tend to mean smaller pigs at birth and therefore a bigger risk that they will die prematurely - either during the farrowing process or shortly afterwards.
Dr Thorup’s presentation to SFT highlighted Danish investigations of another effect, which is the negative correlation between litter size and the weight of the pigs at weaning. This probe in Denmark found that each additional piglet in the litter was associated with a reduction on 150 grams in the weaning weight at 24 days old.
The relationship held true whether the size of the litter went from 5 pigs to 7 or from 13 pigs to 15, he commented.
What is more, supplementary creep feeding of piglets did not seem to compensate for the slower growth of piglets in the larger litters.