Acidity is reckoned to be helpful when feeding piglets. It is needed to support the action of enzymes for protein digestion in the stomach and it promotes a probiotic type of gut flora for the benefit of the animal's health and well-being. There are proven advantages in adding organic acids to reduce the pH of early feeds, to offset the fact that the baby pig, up to and immediately after weaning, secretes only low acid levels. Even so, acidifying the diet is not going to have the expected gain if it is counteracted by a buffering effect from ingredients: in other words, their ability to neutralise acids in the feed.
Buffering by ingredients is most often measured as their acid-binding capacity or ABC. Essentially the measurement is taken by preparing a suspension of the feed sample in water and checking how much of a standard acid must be added in order to reduce the pH to a target level. Ideal components of a piglet starter diet would have a low ABC, indicating that they bind less acid in the stomach and therefore their use promotes greater gastric acidity.
While the theory is secure, feed formulators have complained that ABC is more talked about than specified by the information on individual ingredients available in the scientific literature. There is also a confusion of measurements by different techniques resulting in values without easy conversion factors to compare them. However, an attempt at helping to fill the gap in know-how has now been reported to the Irish Veterinary Journal*. It demonstrates that complete diets with a low acid-binding potential suiting weaned pigs may be assembled by using the ABC values of each ingredient in the formulation matrix.
Examples of ingredients commonly used in piglet starter feeds were obtained from various sources in Ireland and placed in categories according to type. This gave groupings of milk products, cereals, root and pulp products, vegetable proteins, meat and fish meal, medication, amino acids, minerals, acid salts and acids. Although it was no surprise that testing showed the categories to differ significantly on ABC values, the individual ratings within and between these groups were revealing.
Cereals and some root and pulp products had low scores. Acid salts gave the highest rating, followed by minerals. Mineral additives are usually reckoned to be more acid-binding than organic ingredients. But the test in Ireland found great variation between the different mineral types. Highest ABC values within this category were for zinc oxide, limestone flour and sodium bicarbonate. Of the phosphorus sources, defluorinated phosphate was highest on the scale for neutralising ability, dicalcium phosphate and mono dicalcium phosphate had intermediate values, while monammonium phosphate came lowest on ABC.
Organic products gave measurements that correlated positively with their ash and protein contents. Previous investigators have proved that the ABC of a feed rises as its protein content increases. In the Irish examination this was thought to explain why meat meal and fish meal had the highest ratings of all the organic ingredients tested. Milk products (in particular, rennet casein and spray dried skim) also had high ABC values, but a low rating was given to the other ingredients in this category, probably reflecting their lower contents of ash and protein.
Among the vegetable proteins, highest scores went to soybean meal, a proprietary soy concentrate, rapeseed meal and sunflower meal. Maize gluten and milo distillers meal were uncharacteristic of this group in that they both had a pH of under 4.5 and their ABC was relatively low.
The acids category had the lowest ABC ratings of all ingredients, both inorganic and organic. Most values for the individual acids were negative. The most negative among them were for the acids orthophosphoric, fumaric, formic, malic and citric. But the authors of the report caution against employing this list as the sole guide for choosing organic acids. A selection based exclusively on lowering the ABC of the starter feed without having to reduce dietary protein or mineral content might choose orthophosphoric, for example, or fumaric or formic. However, this takes no account of other qualities such as antimicrobial effects, promotion of probiotic bacteria, nutritional value, physical form (dry or liquid) and corrosiveness.
Words of warning are given, too, about considering ingredients by their general category or description rather than by their individual measurements. Evidence came from the study showing, for example, that the geographic origin of an ingredient can affect its ABC. Nevertheless the authors say acid-binding capacity deserves attention in preparing a diet for newly-weaned pigs that can help ease their transition from milk to solid food. Such diets are likely to be especially useful around weaning when a high gastric pH would be a problem for the piglet. They would also fit into a strategy of feeding against E. coli or Salmonella, for those countries are no longer allowed by law to feed low-level antibiotics to their pigs.
(*Measurements of the acid-binding capacity of ingredients used in pig diets, by Peadar Lawlor, Brendan Lynch, Patrick Caffrey, James O'Reilly and Karen O'Connell, in Irish Veterinary Journal 58 (8): 447-452, 2005. The authors are at Teagasc's Moorepark Research Centre and the Faculty of Agriculture of University College Dublin, in Ireland). PIGI