FORAGES AND HEAT STRESS

Classic data from Missouri show that heat stress conditions may reduce rumen pH to levels well below 6.0 – virtually identical to dropping dietary forage from 65 to 35%. Under these low-pH conditions we can expect inefficient rumen fiber fermentation and lower microbial protein output – both of which result in lower milk and milk component production. A

traditional dietary recommendation during heat stress has been to feed less forage and more concentrates in an attempt to lessen the heat load of digestion. However, if we feed a diet that is borderline in fiber and high in starch, that will only exacerbate an already compromised rumen pH and microbial fermentation.

We also know that ration fiber content has a large impact on cow chewing response. As ration fiber increases from about 20 to 25% of DM, cows spend greater time eating, have longer meals, and sort their feed more. Amazingly, 10 to 30% of the energy provided by feed can be used for chewing depending on forage quality. For the heat-stressed cow, feeding forage

that is lower in NDF and higher in NDF digestibility is crucial because it allows the cow to process the feed in less time and expend less energy doing so. Cows may need to spend an extra hour each day to eat higher fiber forages. That hour spent standing at the bunk eating subtracts an hour from some other activity (such as resting) and certainly forces the cow to stand more – potentially placing more stress on her feet during a time when the risk of lameness is already elevated due to the heat stress.

Highly digestible forage NDF will help to reduce the overall heat load on the cow and provide the proper amounts and ratios of VFA to maintain milk and milk component synthesis. But if high quality forage is not available, some research has also indicated that substituting byproduct NDF for the lower digestibility forage NDF can improve milk production during heat stress (i.e. soybean hulls, beep pulp, and similar nonforage sources of fiber). Forage NDF can be reduced from 75% to 60% of total NDF under heat stress with efficient fiber fermentation. A critical component of successfully feeding higher nonforage sources of fiber in place of forage fiber is maintaining a well-formed rumen digesta mat. This mat serves to slow passage of the nonforage fiber sources and will increase rumen NDF digestion by as much as 40% resulting in greater milk and milk component output.

The bottom line is that feeding highly fermentable NDF – from forage and nonforage sources of fiber – will maintain rumen function, VFA profiles that promote milk component output, and

avoid altered pathways of rumen biohydrogenation that depress milk fat. We keep learning more about heat stress effects on cow behavior, feed intake, and health. This new information is useful, but we can’t neglect the basics of heat stress abatement. Ration adjustments (such as increasing digestible NDF) will not be fully effective unless cows are cooled effectively. This

summer, make sure that forage quality and cow cooling go hand-in-hand on your farm!