Feeding a marine-based rumen buffer increases milk production and decreases time of low reticulo-rumen pH in grazing dairy cows offered perennial ryegrass-based pasture

•There is limited research conducted into the use of rumen buffer in grazing dairy cows.•Feeding rumen buffers reduced the amount of time the reticulo-rumen was < pH 5.8 and 6.0.•Feeding rumen buffers increased milk production and affects milk composition.•Feeding rumen buffers increased energy and fat corrected milk yield and milk fat yield.