Effects of replacing barley straw and corn silage by olive cake in the diet on microbial populations in RUSITEC fermenters

Abstract

By-products are an important waste in agro-food industry in many countries, and they can represent an environmental problem, as they are difficult to eliminate. Their use in ruminant feeding could alleviate this problem and contribute to a more sustainable livestock production. Olive cake is a by-product of olive oil extraction that contains bioactive compounds with antimicrobial and antioxidant activities that could also affect the ruminal microbiota, but this aspect has not yet been investigated. Four Rusitec fermenters were used in a cross-over design in two 14-day incubation runs to study the effects of replacing part of the forage by olive cake in a dairy sheep diet on microbial populations. Three rumen-cannulated Merino sheep were used as ruminal content donors for inoculation of the system. Experimental diets were a conventional diet for dairy sheep (50:50 forage:concentrate) with corn silage and barley straw as forage, and a diet in which corn silage and barley straw were partially replaced by olive cake (16.7%). Samples of liquid and solid digesta were collected from the fermenters at the end of each incubation run. DNA was extracted and the abundance of bacteria and protozoa, as well as the relative abundance of fungi and archaea, were assessed by qPCR. Bacterial diversity was analysed using Automated Ribosomal Intergenic Spacer Analysis (ARISA). Data were processed using R with the vegan package and the PROC MIXED from SAS. The number of peaks detected in the ARISA electropherograms and the Shannon’s index were not affected (P>0.05) by the inclusion of olive cake in the diet, but they were higher (P=0.006 and P=0.005, respectively) in liquid than in solid digesta. Principal coordinates analysis based on Bray-Curtis distance showed that samples were not clearly grouped by diet, due to the variability of samples from the same diet and the strong effect of the digesta phase. Bacterial and protozoal abundance in the liquid digesta and the relative abundance of fungi and archaea in both digesta phases were not affected (P>0.05) by the inclusion of olive cake in the diet. In the solid digesta, diet affected (P=0.013) the bacterial abundance and tended to affect (P=0.06) the protozoal abundance, both being higher for the diet containing olive cake than for the conventional diet. In conclusion, the addition of olive cake in substitution of corn silage and barley straw in the diet promoted greater bacterial and protozoal growth in the solid digesta of Rusitec fermenters