Review: Role of silage volatile organic compounds in influencing forage choice behavior and intake in ruminants

Abstract

Silage volatile organic compounds (VOC) can have a negative, neutral, or sometimes positive influence on silage preference and intake in ruminants. We have reviewed the literature on silage VOC effect on ruminant feeding behavior since 1950 and have discussed the various mechanisms of action and magnitude of the impact of VOC, such as acetic acid (AcA), propionic acid (PrA), butyric acid (BtA), ethanol, esters, and propanol, on ruminant feeding behavior. Although it has been known for some decades that fermentation products or VOC reduce dry matter intake (DMI), the significance of each compound is still unclear. Due to the complex silo environment and the concurrent formation of different fermentation products or VOC, an error-free relationship between a specific compound and DMI may not be possible with silage DMI models. Testing a specific VOC on preference and DMI in feeding trials can provide a better picture than infusion studies because the former method can check for both sensory and post-intake factors that influence preference and DMI. Although previous results were somewhat ambiguous, it seems that the simultaneous presence of high amounts of different VOC reduced preference and DMI more than a single compound. Among VOC, AcA, and PrA seem to have more hypophagic effects than others. However, in many studies the negative effect of AcA and PrA on DMI was not observed. Nonetheless, these compounds can offset their drawbacks in some cases. For instance, aerobic spoilage, which can have a significant negative impact on preference and intake, can be reduced by the presence of AcA and PrA in silage. Although anecdotal evidence suggests that alcohols (ethanol and propanol) and esters (ethyl acetate and ethyl lactate) can reduce preference and DMI, the addition of these substances in preference and feeding trials does not show negative effects. Other VOC, such as BtA, methanol, iso-BtA, and iso-valeric acid, seem to have no negative influence on preference or DMI. Metabolomic studies revealed that thousands (∼6000) of metabolites, among which many are VOC, can be found in silage. Future experiments that employ metabolomic analysis, in addition to routine chemical analysis, may have the potential to reveal other unidentified VOC that can impact silage preference and intake. Discovering these substances and developing proper management techniques to control them may improve both the productivity and profitability of ruminant production.