Acquisition of innate preferences may already take place before birth by transmission of chemosensory cues from the maternal diet to the offspring through the amniotic fluid. Some flavour compounds enter the amniotic fluid and seem to be perceived by the foetus either through fluid ingestion or through the maternal blood in the olfactory epithelium via the placental blood circulation. The group of the Adaptation Physiology of the Animal Science Department in Wageningen has been working on the vertical transmission from the sow to their offspring. Their findings suggest that a familiar flavour from the sow’s diet in the postweaning diet reduces neophobia for this food and generally reduces stress, resulting in a higher performance and welfare of piglets after weaning (Oostindjer et al., 2009).
Novel ingredients evoking bitter taste elicit a strong neophobic feeding behaviour also in adult animals. Bitter taste has been related to naturally occurring compounds mainly of plant origin (i.e. anti-nutritional factors –ANF-) and may lead to behavioural aversions to feed. Traditional feed ingredients used in pig diets (such as corn, wheat, rice, milk whey, soybean and protein concentrates) have been selected for being low or free of ANF content but their market prices may fluctuate and reach costs that may force animal nutritionists to consider the utilization of alternative protein and energy sources in which palatability is likely to play a limiting role and trigger strong neophobic responses (e.g. rape, peas, DDGS, sunflower and sorghum). Pigs have been reported to depress feed intake presumably due to plant ANF present in wheat, soybean, sorghum, rapeseed and canola meals, peas, sunflower meal, DDGS and plant extracts.
For example, preferences for cereals and protein sources in piglets have been recently reported after the PhD thesis of David Solà-Oriol (2008). Among the 15 protein sources tested, potato protein, rapeseed and sunflower meals showed the lowest values of acceptance, presumably related to their content in alkaloids, glucosinolates and lignin among other ANF. On the other hand, ingredients releasing stimulating umami (free amino acids), sweet (high lactose levels) or salt (high Na levels) tastes in absence of putative bitter compounds showed the highest preference values. Tokach et al. (2003) identified a few ingredients having a direct positive impact on feed intake in piglets: dried whey and to a lesser extent other lactose and carbohydrate sources, whey protein concentrate, spray-dried animal plasma and blood meals, dried porcine solubles and high quality fish meal. In general, the most palatable ingredients in pig diets have been found to be derived from animal tissues, ANF or toxic free compounds and are a highly digestible energy or protein source with significant fractions of putative salty or sweet and/or umami tastants.
Substitution of high quality and highly preferred ingredients in piglet diets has been addressed by several authors. High quality lactose sources (dried whey) and spray-dried animal plasma may only partially be substituted by other good quality ingredients in piglets. Our research group has been heavily involved in the development and application of new flavours to improve palatability and feed intake in piglets. We have been successful in proving the effectiveness of selected flavours in masking some low palatability ingredients such as rapeseed and sunflower meals by preventing strong neophobic responses. Furthermore, milk derivatives (dried whey) and cooked cereals were totally or partially substituted by non-cooked cereals and soybean and rapeseed meals without an impact on piglet performance except when a selected flavour was added.
In summary, we propose several strategies that may help improve feed intake in swine based on palatability:
1- Piglet conditioning by early maternally driven exposure to tastes and smells.
2- Hull removal and lowering ANF content of feeds and feed ingredients.
3- Ingredient technology (cooking, enzymatic treatments…) that facilitates either oral nutrient release such as glucose or some savoury amino acids, or inactivates ANF ingredients.
4- Use of high preference flavours to enhance early feed intake in the piglet or to mask dietary changes and use of alternative feed ingredients.