From late Dec. 2003 through Feb. 2004, a 2500 cow central New York dairy experienced an outbreak of displaced abomasums among the entire milking herd. Due to the nature of the outbreak and group management, the herd was divided and evaluated as two separate groups: fresh cows (<30 DIM) and later lactation animals (>30 DIM). The fresh cows experienced more than double the annual average number of DAs during Jan. and Feb. while the later lactation animals experienced a spike in DAs during Jan. which decreased but remained above the annual average for the year in Feb. After many weeks of ration changes and no break in the increased number of DAs, it was finally recognized that the peNDF (physically effective fiber) had been overestimated in both the BMR (brown midrib) and the regular corn silage. Both silages were of inconsistent and somewhat poor quality due to flaws in harvest and packing. More fiber was added to the rations and the BMR corn silage was removed from the ration while the poorly packed regular corn silage was eventually fed out. DA numbers returned closer to the annual average for both fresh and later lactation animals in Mar. 2003.