Effect of dietary dry matter concentration on the sorting behavior of lactating dairy cows fed a total mixed ration.

The objective of this study was to determine whether addition of water to a high-moisture total mixed ration reduces feed sorting by dairy cattle. Twelve lactating Holstein cows, individually fed once per day, were tested on 2 diets in a crossover design with 21-d periods. Diets had the same dietary composition and differed only in dry matter content, which was reduced by the addition of water. Treatment diets were 1) dry (57.6% DM) and 2) wet (47.9% DM). Dry matter intake (DMI) and milk production were monitored for each animal for the last 7 d of both treatment periods. For the final 3 d of each period, milk samples were taken for composition analysis and fresh feed and orts were sampled for particle size analysis. The particle size separator had 3 screens (19, 8, and 1.18 mm) and a bottom pan, resulting in 4 fractions (long, medium, short, fine). Sorting was calculated as the actual intake of each particle size fraction expressed as a percentage of the predicted intake of that fraction. Contrary to the hypothesis, cows sorted the wet diet more extensively than the dry diet. Sorting of the dry diet was limited to a tendency to refuse short particles, whereas the wet diet was sorted against long particles and for short and fine particles. Water addition reduced DMI, neutral detergent fiber intake, and starch intake of cows on the wet diet. Increased sorting on the wet diet resulted in a tendency for decreased concentration of dietary neutral detergent fiber consumed and also resulted in increased starch concentration of the diet consumed. Milk production and components were unaffected by treatment. Our results suggest that water addition to high-moisture total mixed rations, containing primarily haylage and silage forage sources, may not be an effective method to reduce sorting. Furthermore, water addition may negatively affect DMI and encourage sorting, resulting in the consumption of a ration with different nutrient composition than intended.