Complex sentence: After the peak, production declines with or without application of Posilac, but declines more slowly with it than without, which permits dairy cows to produce more milk over the span of a lactation.
Split into simpler sentences: After the peak, production declines with or without application of Posilac, but declines more slowly with Posilac than without. This decrease in the rate of production decline permits dairy cows to produce more milk over the span of a lactation.