Associative Effects between Forages and Concentrates on In Vitro Fermentation of Working Equine Diets

SIMPLE SUMMARY: The aim of this paper is to evaluate post-gastric changes in the fermentability of mixtures of different forages and concentrated feeds that are representative of the traditional diets of high-performance horses using the in vitro gas production (GP) technique. Based on GP and volatile fatty acids (VFA), the post-gastric fermentation of concentrates is greater than that of forages. However, when we combined forages and concentrates, the GP parameters and VFA concentrations of some forage–concentrate mixtures had unexpected values in comparison to the fermentation of pure ingredients, indicating the occurrence of associative effects. This shows that there is a need to evaluate the fermentation of diets, rather than predicting from the values of pure ingredients. ABSTRACT: Background: Horses are hindgut fermenters, and it is therefore important to determine the postgastric nutritive value of their feedstuffs and diets. Moreover, it has been demonstrated in other animal species that the fermentation of diets results in different values than those expected from pure ingredients. Therefore, the general objective of this work is to evaluate the gas production (GP) and volatile fatty acid (VFA) concentration, as well as the associative effects, of mixtures of different forages and concentrated foods, which are representative of the traditional diets of high-performance horses. Methods: An in vitro gas production experiment was conducted to assess the fermentation of two forages and three concentrates that are typical in horse diets. The combination of 70% of forage and 30% concentrates was also assessed to determine potential associative effects. Results: Concentrates and grains produced higher GP and VFA than forages when evaluated alone. When experimental diets were incubated, GP parameters and VFA concentrations of forage–concentrate mixtures had unexpected differences from the values expected from the fermentation of pure ingredients, suggesting the occurrence of associative effects. Conclusions: Our results indicate that there is a need to evaluate the fermentation of diets, rather than predicting from the values of pure ingredients.