Rumen microbiota enable dairy cattle to breakdown fiber into useable energy for milk production. Rumen bacteria, protozoa, and fungi ferment feedstuff into volatile fatty acids (VFA), the main energy source, while methanogens utilize fermentation by-products to produce methane. Milk fat contains several bioactive rumen-derived fatty acids (FA), including odd-chain FA (OCFA) and branched-chain FA (BCFA), important for maintenance of human health. The overarching dissertation goal was to determine which factors affect rumen methanogen and protozoal community structures and their metabolism products, while defining relationships between rumen microbiota and animal performance. Results presented contribute to the goals of providing new knowledge to dairy farmers, maintaining ruminant health, and enhancing bioactive FA in milk.
The first objective was to use next-generation sequencing techniques to determine if lactation stage and dairy breed affect rumen methanogen and protozoal community structures and protozoa cell FA compositions in Jersey, Holstein, and Holstein-Jersey crossbred cows at 3, 93, 183, and 273 days in milk (DIM). A core methanogen community persisted by lactation stage and breed. At 3 DIM, methanogen 16S rRNA gene sequences formed distinct clusters apart from 93, 183, and 273 DIM, reflective of the dietary transition period post-partum. The starch-utilizing protozoal genus Entodinium, was more abundant in Holsteins than in Jerseys and Holstein-Jersey crossbred cows and positively correlated with milk yield. Jerseys had greater iso-BCFA contents in protozoa and milk and protozoa of the genus Metadinium.
The second objective was to determine if supplementation of mixed cool-season grasses with annual forages (AF) alters the forage, microbial, and milk FA contents during typical periods of decreased pasture growth in Northeastern US. In short-term grazing (21d) of AF, ruminal VFA and major rumen-derived FA were not altered in bacterial and protozoal cells, suggesting little alteration of biohydrogenation and maintenance of ruminant health. In spring, milk contents of iso-15:0 and 17:0 per serving of whole milk were greater in control (CON)-fed cows, while contents of 12:0 and 14:0 per serving were greater in AF-fed cows. Contents of de novo FA and OCFA per serving of whole milk were greater in summer AF-fed cows than CON-fed cows, while total contents and BCFA did not differ, suggesting post-ruminal FA modifications in adipose tissue and the mammary gland.
gland. The third objective was to characterize and relate the rumen microbiota from CON- and AF-fed cows to animal performance. Rumen protozoal taxa were not altered, while less abundant bacterial taxa (< 5%) were different in both periods. The protozoal genus Diplodinium was positively correlated with feed efficiency and milk fat yield. In spring, AF-fed cows had greater abundances of the methanogen species Methanobrevibacter millerae, whereas CON-fed cows had greater abundances of the methanogen species Methanobrevibacter ruminantium, potentially as a result of differences in substrate availability. In conclusion, the work presented identifies several factors that influence rumen microbiota, rumen microbial FA, and milk FA, while providing new information to dairy farmers, researchers, and consumers.