Particle Size and Stocking Density: Can Heifers be Affected Too?

Until the onset of her first lactation a dairy heifer doesn’t make any money for the farmer; in fact, she costs a considerable amount of money. A recent report by Penn State Extension estimates that a heifer costs $1.17 per day from 6 months of age until breeding and $2.02 per day from breeding until she is in her prefresh stage. A lower age at first calving results in higher milk yield and profit. A combination of correct feeding and management techniques is necessary to achieve the goal of 82% of a heifer’s mature body weight at the time of calving at a lower age. Research by Coblentz et al. (2017) at the University of Wisconsin evaluated the effects of straw processing and pen stocking density on sorting behaviors of the diets offered and growth performance of Holstein heifers. The study looked at short straw versus long straw inclusion in otherwise identical diets fed to heifers stocked in densities of 100, 125, or 150%. This research is similar to what has been done here at Miner Institute with lactating cows.

Heifers fed the short straw diet and housed at 125% and 150% stocking density had a reduced total weight gain and average daily gain compared to heifers fed the same short straw diet but housed at 100% stocking density. There was no difference observed between the two overstocked pens (125 and 150%). Since there was no difference in dry matter intakes between stocking densities for the short straw treatment, the reduced weight gain can be explained by a reduced feed efficiency in the overstocked pens. Unlike the short straw diet, there were no differences in any measurement of growth or feed efficiency between heifers in the 100% stocking density pen compared to heifers in the overstocked pens for the long straw diet.

Using a Penn State Particle Separator, Coblentz was able to determine sorting behaviors by sampling the refusals at the bunk at time points throughout the day and comparing the particle size fractions to those of the TMR that was initially fed. Although both the heifers fed the short straw TMR and the heifers fed the long straw TMR sorted against long particles (>19mm) and in favor of medium, short, and fine particles (<19mm), more aggressive sorting was exhibited by heifers on the long straw treatment (as seen in the figure). There was no difference in sorting behaviors between stocking densities, however the sorting behavior potentially becomes more detrimental in overstocked conditions. Social tension between dominant and subordinate heifers is exacerbated when feed bunk space is limited, which would lead subordinate heifers being forced to wait until the more dominant heifers are done before they can eat. Not only does this mean the subordinate heifers are spending more time standing idly while they wait for their turn to eat, but when they finally are allowed to eat they’re presented with feed that was already sorted, leading to different nutrient intakes than the TMR was formulated to supply. Understandably this would lead to a greater variation in growth rates within the pen.

As with lactating cows, heifers experience stressors and have a limited ability to cope before performance (i.e. growth rates) becomes impaired. Therefore, we must pay attention to the management of our heifers as we would our lactating cows. Feeding high quality forages that don’t encourage sorting and keeping pens at an appropriate stocking density are among the management practices that will help maximize growth performance and lead to an earlier age at first calving and a more productive dairy cow.