Replacing Forage by Crude Olive Cake in a Dairy Sheep Diet: Effects on Ruminal Fermentation and Microbial Populations in Rusitec Fermenters

SIMPLE SUMMARY: Large amounts of olive cake are generated after olive oil extraction, representing an environmental pollutant whose disposal is highly expensive for olive-processing companies. Using olive cake in ruminant feeding is a possible alternative, as it is a good source of fiber and contains polyphenols, although its nutritive value needs to be properly assessed. In this study, we evaluated the effects of replacing 33% of the forage (maize silage and barley straw) by olive cake (16.6% of total diet) in a mixed diet for dairy ruminants on in vitro ruminal fermentation using Rusitec fermenters. Olive cake inclusion increased diet degradability, whereas rumen fermentation, microbial growth, and microbial populations were not modified. Results indicate that maize silage and barley straw could be partially replaced by olive cake in a diet for dairy sheep without compromising rumen fermentation. The use of olive cake in ruminant diets could be especially indicated in periods of forage scarcity. ABSTRACT: Olive oil extraction generates large amounts of a highly pollutant by-product called olive cake (OC), and its use in ruminant feeding could be an alternative. This study was designed to evaluate the effects of partially replacing forage by crude OC (COC) in a mixed dairy diet on rumen fermentation and microbial populations in Rusitec fermenters. The COC replaced 33% of the forage (66% maize silage and 33% barley straw) and was included at 16.6% of the total diet. Four fermenters were used in a cross-over design with two 13-day incubation periods. Experimental diets had a 50:50 forage-to-concentrate ratio and were formulated to contain the same protein (16.0%) and neutral detergent fiber (32.5%) levels. Compared with control fermenters, those fed the COC diet showed greater (p ≤ 0.02) pH (6.07 vs. 6.22), diet disappearance (0.709 vs. 0.748), and butyrate proportions (18.0 vs. 19.4), but there were no differences in volatile fatty acids and ammonia production. Microbial growth, bacterial diversity, protozoal abundance, and relative abundance of fungi and archaea were unaffected by diet, although the solid phase of COC-fed fermenters showed greater (p = 0.01) bacterial abundance than control ones. Results indicate that COC could replace 33% of the forage in a mixed dairy diet.