The good taste of pigs (part IV): bitter

Bitter taste has evolved to detect potential toxins in the foods, what can be seen as one of the most effective mechanisms of self-protection. Bitterness leaves an unpleasant feeling linked to a decrease in feed consumption. However, not all bitter substances are toxic since some plants have learned to exploit bitter sensing to their advantage. Some plants synthesize non-toxic compounds that trigger bitterness solely to avoid being eaten. Bitterness is relevant to commercial pig feeding. For example, pigs are four times more sensitive to dietary glucosinolates from rapeseed meal than cows (du Toit et al. 1991). However pigs seem to adapt rapidly to non-harmful bitter tastants and after an initial drop in feed intake they recover feed consumption only after a few days. There are two potential mechanisms by which bitter-rich ingredients may result in changes on feed intake: 1-mechanisms directly related to palatability (bitter tasting compounds) or 2-gut defence mechanisms.

1- Bitter tasting compounds in pig feeds (palatability)

In practical pig feeding a wide array of toxins and anti-nutritional factors (ANFs) many of which are perceived as bitter may be present. The detection of these undesired compounds through the taste system will cause decreases in feed consumption. For example agricultural practices include the use of significant amounts of chemicals such as pesticides but their potential impact of their residues on feed intake has not been studied. Similarly, mycotoxins are very common in stock feed and some result in reduced appetite (i.e. fusariotoxins and aflatoxins); however, it is not clear if the sensorial impact is due to its bitterness. In contrast, several common feed ingredients have been reported to affect feed intake through bitterness. They are plant derived compounds or feed additives.

1.1 Plant derived compounds

The ANFs are naturally occurring compounds synthetized by plants that affect the use dietary nutrients or the health of the animal when ingested. In general, ANFs has evolved as defensive mechanisms developed by plants to protect against predators. Some of them will have deleterious health effects but some are apparently innocuous. In a recent review, Clasadonte and van der Poel (2009) did not find a linear relationship between the level of ANFs with changes in voluntary feed intake in pigs. Their conclusion outlines the limited research available examining the effects of isolated ANFs. Research on feed intake in pigs commonly uses complex diets that contain a large variety of constituents and under these circumstances, sensorial perception is not discriminative to only ANFs.

However, there is a long list of ANFs that have been found to instinctively decrease feed intake by reducing its palatability such as trypsin inhibitors, lignin, glucosinolates, gossypol or alkaloids (Table 1). Reportedly, anyone or several of these and other ANFs might be present in commercial pig feed through dietary inclusion of feed ingredients shown in Table 1. For example, lectins present in raw soybean and beans (Phaseolus) have a very high impact on feed intake. Sola-Oriol et al (2011) found very low preferences for sunflower, rapeseed and potato protein presumably linked to their chlorogenic acid, gucosinolates and alkaloid content.

Additional and uncontrolled sources of potential toxic and bitter plant derived compounds in feeds are due to weeds and weed seed contaminants. Weed seeds are prevalent contaminants of feed grains and are responsible for significant livestock losses, but there is a lack of screening methods for identification of these contaminants (van Barneveld, 1999). In addition, their impact on feed intake in pigs has not been studied.

Table 1: ANFs in feed and negative impact on feed consumption or feed preference in grower pigs (sources: D’Mello, 1995; Clasadonte and van del Poel , 2009; Roura , 2012; Sola-Oriol et al., 2008, 2011).

1.2 Feed additives

Low inclusion feed ingredients (i.e. feed additives) have often strong effects on feed intake that may involve taste. For example, many plant extracts and most drugs, such as antibiotics, seem to be bitter to pigs (Nelson and Sanregret, 1997; Windisch et al., 2008). Tilmicosin has been found to elicit a strong electrophysiological response in pig taste nerve fibers. In addition, a wide range of pharmaceuticals (mainly related to analgesic, antihistaminic and antiseptic activities) as well as some chemical compounds such as quinine HCl, denatonium, caffeine and sucrose octaacetate also elicited avoidance responses in pigs (Danilova et al. 1999; Nelson and Sanregret 1997). Bitrex (the harmless but bitterest compound known to humans) abruptly reduced feed intake in pigs. However, the reduced consumption was back to normal after only a few meals (Blair and FitzSimons 1970) since did not cause any discomfort to the animals.

2- Gut defence mechanisms as a response to bitter compounds

Bitter taste perception in the oral cavity results in an immediate decrease in the food ingested. However, a part of the undesired substances inevitably makes it through the oral cavity further down the gastrointestinal tract (GIT). Thus, the protective mechanisms present in the GIT are fundamental to prevent damage of the potential toxins. The presence of taste sensory cells in the epithelium of the GIT functions as a chemosensing network that monitors nutrients and toxins initiating a cascade of events that will result in either absorption (nutrients) or excretion (toxins) of the intestinal contents. In the case of bitter sensory cells present in the GIT mucosa have been reportedly identified in several defence mechanisms:

3- How to use bitterness to our advantage in commercial pig feeding

In general, the physiology behind bitter taste perception in the tongue and in the rest of the GIT shows promising commercial implications related to modulation of feed intake.

For example, in weaner diets it is clear that the use of ingredients that trigger potential bitter taste needs to be avoided (even traces of alkaloids, sinapines and glucosinolates will have a negative impact on feed intake in piglets). That also includes medicated feeds. When feeds need to be medicated, it should be kept in mind that most antibiotics will be bitter and will cause an initial negative response on feed intake. In this case, strategies to mask the bitterness of antibiotics should be approached (e.g., encapsulation, proper drug formulation embedded in sugar, etc.) and may have a quick return on the investment.

On the other hand, modulating feed intake with bitter compounds might be an interesting tool to limit excessive voracity of pigs and sows at certain stages. However, having a good control of that practice will require further research.