Particle size matters for high-straw dry cow diets by Casey Havekes and Trevor DeVries for Progressive Dairy

In the weeks leading up to calving and the early postpartum period, various behavioral, metabolic and physiological challenges are imposed on the dairy cow.

One of the biggest stressors the cow faces at calving is the sudden and dramatic increased nutrient demand for milk production, which often coincides with a lag in dry matter intake (DMI). Considerable research has been focused on identifying fresh cow feeding strategies to promote greater intake in early lactation, thereby reducing the severity of negative energy balance cows experience and, thus, reducing the risk of metabolic and infectious disease. However, more recently, we have learned the diet consumed by dry cows may be equally important in terms of stimulating intake in early lactation.

More and more commonly, dairy producers are feeding high-straw, controlled-energy dry cow diets, more often referred to as the "Goldilocks Diet," which is based on the concept of not too much and not too little. These diets incorporate low-nutrient-dense ingredients, such as wheat straw, into the diet - and are designed to allow the cow to maximize intake, particularly in volume, while controlling for energy consumption. From a physiological standpoint, these diets may work great because cows are able to eat as much as they want without gaining excessive body condition. However, from a feeding behavior standpoint, issues arise when the particle distribution of the diet is considered.

Straw is a dry, bulky forage, and high inclusion rates may limit DMI, as passage rate through the rumen may be slowed, particularly when particle size is long. From a management standpoint, a big challenge on-farm with feeding high-straw diets is achieving a particle size small enough to reduce sorting. Sorting occurs when cows selectively consume one portion of the diet relative to another portion, leading to the consumption of an imbalanced diet. Not only does sorting compromise the nutritive value of what that cow consumes, but it also may reduce the nutritive value of the feed that remains in the bunk throughout the day when perhaps more subordinate animals feed.

This results in inconsistency in what cows consume, both within and between cows. Overall, the goal of these high-straw dry cow diets is to promote consistent consumption; thus reducing feed sorting and promoting large volumes of feed consumption are crucial components to the success of the feeding program.

Our research group at the University of Guelph recently conducted a study to address the behavioral feeding issues with these high-straw dry cow diets. We set out to answer the following question: Can we improve intake, reduce sorting, promote overall health and productivity of transition cows by manipulating the particle size of straw in a high-straw dry cow diet?

Cows entering their second lactation or greater were enrolled in a study at dry-off, about 45 days prior to calving. Upon enrollment, cows were fed a dry cow TMR that contained (on a dry matter basis) 36% corn silage, 35% dry cow pellet and 29% wheat straw. Prior to adding to the TMR, the straw was chopped in a tub grinder; for half the cows a 2.54-centimeter (1-inch) screen was used and, for the other half of cows, a 10.16-centimeter (4-inch) screen was used. After calving, all cows were fed the same lactating diet regardless of pre-calving dietary treatment.

During the dry period, cows fed the shorter-straw diet had greater feed intake compared to cows fed the longer-straw diet. Perhaps even more interesting was the change in intake we observed as cows approached calving. During the week leading up to calving, cows fed the shorter-straw diet were able to maintain more consistent intake compared to cows fed the longer-straw diet, who exhibited a faster decline in intake during that week. Cows fed the longer-straw diet also sorted that diet more extensively against the longest dietary particles than cows fed the shorter-straw diet.

In past research, it has been demonstrated that cows will sort a diet when the particles are more easily distinguishable from one another, as was the case with the longer chopped straw diet. The most obvious consequence of sorting was that those cows consumed a diet which was not nutritionally intended for them, but also the increased sorting against the long particles resulted in a slower feeding rate and may have limited their ability to maximize total dry matter consumed.

Interestingly, cows fed the shorter-straw diet sorted against the finest dietary particle fraction, whereas cows fed the longer-straw diet did not sort for or against this fraction. This finding was surprising, as cows will typically sort in favor of the smaller particles of their diet, which are often the most palatable. However, in the case of our dry cow diets, the finest particles in the diet were mostly straw fines and dust, and the shorter chopped straw diet contained even more straw fines and dust than the longer chopped straw diet. Because it is likely that cows do not like straw fines and dust, they sorted against this fraction. Based on this finding, we would recommend using a straw chopping technique that minimizes overprocessing the straw, thereby reducing the amount of fines and dust in the TMR.

In recent years, many researchers have investigated how pre-calving feeding behavior influences post-calving success. One finding of particular interest from this work is the association of increased risk of developing ketosis with greater drops in intake as cows approach calving. In support of this finding, in our research cows fed the longer-straw diet had greater blood beta-hydroxybutyrate (BHB) levels three weeks post-calving, compared to cows fed the shorter-straw diet.

We also determined that cows fed the longer-straw diet had a greater maximum level of BHB, indicating these cows may have been experiencing a greater extent of negative energy balance in early lactation. It is also noteworthy that cows fed the shorter-straw dry diet had improved rumen health in the first week post-calving, as evidenced by less rapid drop in reticulorumen pH during that time period. This is likely related to cows fed the shorter-straw diet having more consistent intake, both in amount and composition, in the week leading up to calving, which potentially helped better prepare the rumen for the transition onto the more fermentable, higher-grain lactating diet they were fed at calving.

Overall, the results from this research suggest that maintaining a short straw particle size in a high-straw dry cow diet can play a significant role in improving intake, reducing feed sorting and improving metabolic health. Based on these results, we recommend evaluating the particle distribution of your dry cow diet (ask your nutritionist to perform a particle size analysis on your farm) - the longest forage particles should not make up more than about 5% of the diet, which can, in part, be accomplished by chopping your dry forages shorter. In the long term, also consider evaluating the chop length of your ensiled forages, as these will also contribute to your overall diet particle size distribution.  

Casey Havekes completed her Masters of Science at the University of Guelph and is currently a regional dairy specialist with Cornell University Cooperative Extension.

Trevor DeVries is a professor at University of Guelph. Email Trevor DeVries.