Three studies were conducted on early to late lactation Holstein dairy cows to examine the effects of forage particle size (FPS) and ration sorting on chewing behavior, ruminal fermentation, and milk yield and components. The objective of the first experiment was to study effects of replacing alfalfa haylage with dry chopped alfalfa hay in the ration on sorting activity and to determine effects on ruminal fermentation, milk production, or milk composition. In addition, a second objective of this study was to compare results of the PSPS and RTPS for the same TMR samples and to determine effects of separation method on particle size distribution. Ration FPS was varied by replacing alfalfa silage with dry chopped alfalfa hay. The levels of hay used were 5, 10, 20, and 40% of forage DM. The results of this study showed that sorting occurred in all rations, but there was only minimal difference in the type or degree of sorting between treatments and only during the first 4 h after feeding. Sorting activity was highest at the beginning of the d and by 24 h after feeding the diets consumed by the cows were not significantly different from the offered diets. There were no negative effects of including dry chopped alfalfa hay in rations up to 23.5% of ration DM on DM intake, milk yield, and rumen fermentation. Small decreases in milk fat and protein content were found to occur with increasing dry hay inclusion. Data from the Penn State and Ro-Tap particle separators were compared, when separating the same TMR samples, and it was determined that data obtained from these 2 methods of particle separation are not directly comparable and that method of particle separation should be considered when interpreting experimental results.

The second experiment’s objective was to study the interactions between FPS and ruminally fermentable carbohydrates (RFC) for ration sorting, ruminal fermentation, chewing activity, and milk yield and components. This study varied FPS and RFC by feeding 2 lengths of corn silage and 2 grind sizes of corn grain. The results showed that altering RFC had greater influence on milk production parameters than FPS; increasing RFC increased milk yield and protein content and decreased milk fat content. Ruminal fermentation was not affected by either FPS or RFC. Ration sorting occurred on all diets as evidenced by the changes in starch, NDF, and particle size composition of the refusals throughout the d and also by selection indices. Diets containing long FPS were sorted to a greater degree than diets containing short FPS, but there was no interaction between FPS and RFC for ration sorting. There was an interaction between FPS and RFC for DMI; DMI decreased with increasing FPS when the diet included low RFC and did change when the diet included high RFC and DMI increased with RFC for the long diets and did not change with RFC on the short diets. Finally, it was determined that approximately 5% of fecal particles were greater than 6.7 mm and that this may be a more accurate estimate of the critical particle size for rumen escape in modern lactating dairy cows.

The objective of the final experiment was to induce a bout of SARA in lactating dairy cows that had ad libitum access to 2 distinct diets that varied in FPS and starch fermentability and to determine how SARA affects TMR selection in dairy cows. One diet consisted of long corn silage and dry cracked corn and the other diet consisted of short corn silage and dry fine ground corn. When offered these 2 diets simultaneously cows consumed 18.1% of their total daily intake as long FPS and low RFC diet. However, after a bout of subacute ruminal acidosis, cows increased their intake of the longer ration to 38.3% of total daily intake. The following d long ration intake moderated to 28.0% and 2 d after the acidosis bout intakes were back to normal at 18.6%. These results indicate that cows are able to alter their diet preference for higher physically effective fiber and slower starch fermentability during a bout of subacute ruminal acidosis, and that they can effectively recover from this type of SARA within 72 h when appropriate diets are available.