Volatile fatty acids in ruminants and their role as feed additives: a review

Ruminants, as an inseparable part of the food chain, will play a determining role in meeting future food demands and guaranteeing global food security for the ever-growing population. In this regard, supplying ruminants with feed material and feed additives that enhance animal health and products has attracted great research interest in the field of ruminant nutrition. Volatile fatty acids (VFAs), such as acetic, propionic and butyric acids, which are endogenously generated in the rumen, are mainly responsible for providing the animal with metabolizable energy and as precursors for animal products. In addition, VFA can be supplied to ruminants as ex situ additives. Extensive research on VFA additives has proven that VFA supplementation affects the development of ruminant gastrointestinal tract, hormone levels, milk yield and composition, adipose tissue, meat quality, weight gain, dry matter intake, etc. The current review provides a unique collection of studies on the effects of VFA additives on ruminants in addition to an in-depth overview of the ruminant digestion system and the fate of different fractions of feed material. Moreover, the potential for sustainable provision of VFA additives from organic-rich waste and residues through anaerobic digestion plus post separation and concentration is critically discussed in this review.