How low starch digestibility will affect the summer milk slump

By Wyatt Smith, Vita Plus dairy specialist
Much of the Upper Midwest has noted lower-than-normal starch digestibility in this year’s corn crops. Conversations have focused on the ensiled starch sources on farm, including corn silage, high-moisture corn and snaplage. With warm weather only a few months away, we’ve started to question what impact this lower starch digestibility will have on the traditional summer milk slump.  To better understand what to anticipate, it’s worth reviewing what influences starch digestibility.

Factors affecting starch digestibility
Both the physical and chemical characteristics of corn kernels influence starch digestibility.

In terms of physical characteristics, the key factor in starch digestibility is particle size. When processing kernels down in size, the seed coating on the exterior of the kernel is broken, allowing rumen bacteria to attach to the inner starch.  Additionally, by reducing the particle size, the surface area increases, allowing for greater bacterial activity and, thus, starch digestibility.

The chemical properties of kernels relate to how vitreous the corn is. In other words, how hard and protected is the starch within the kernel? Much of this relates back to the growing conditions and the dry matter content of the plant when it was harvested. Within the corn kernel, starch is encapsulated in a matrix made of prolamin proteins. This is critical because the protein matrix hinders starch availability to the rumen bacteria.  The protein matrix is broken down through fermentation and bacterial activity during the ensiling process. As storage time increases, the protein matrix degrades further, allowing for greater starch digestibility.

Looking ahead to summer
Now let’s think about summer heat stress and its associated milk slump.

Can we identify potential starch digestibility bottlenecks on farm? Is one starch source an issue while others are not? Tools like lab tests, seven-hour in vitro starch digestibility, kernel processing score, and fecal starch can confirm which ingredient is potentially limiting starch availability in the rumen.

When analyzing the starch sources ensiled on farm, a longer storage length (greater than eight months) works in favor of starch digestibility. Next consider whether each of the starch sources processed to an ideal degree.  If not, is additional processing an option? If the high-moisture corn is on the drier side and not well-processed, can it be reground during feedout to increase surface area for ruminal bacteria to attach to and utilize to a greater degree? Unfortunately, additional processing is not an option for under-processed corn silage kernels as it would alter the fiber fraction of the silage as well. With that in mind, it is critical to think ahead to the next corn silage harvest and how you will achieve adequate kernel processing.

If altering current starch sources for improved starch digestibility is not possible, the next option is to purchase other starch sources to achieve the desired ruminal starch and corresponding performance. When incorporating new starch sources with ration changes, the lab tests stated above will aid in assessing the effect of each change.

In summary, we are facing a potential for decreased starch digestibility, leading to lower-than-target performance, especially when coupled with summer heat stress. Take the time to evaluate the current starch sources individually and assess potential changes to mitigate the risk of milk production loss. Finally, think about what heat stress abatement strategies can be used to reduce summer heat stress and the prevalence of the summer milk slump.  Work with your nutrition consultant to determine what strategies best fit your herd.