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Beating the Summer Slump – Keeping Your Cows Cool (part 2 of 2)

Paul Porter, Ph.D, Land O’Lakes

With the warm spring we are experiencing, it is apparent that we will need to develop strategies to deal with heat stress in order to minimize the negative effects of heat on production and reproduction. Our column two weeks ago talked about ways to modify the environment to keep cows cool. This week we will look at ways to modify the diet of lactating dairy cows to improve dry matter intake during heat stress.

Heat stress occurs when the cows heat load exceeds her capacity to lose heat. Heat stress is a function of both temperature and relative humidity. The greater the relative humidity, the lower the temperature at which heat stress occurs. For example, at 80% relative humidity, mild heat stress occurs at 72 degrees F. As temperatures increase, the cow strives to maintain an internal temperature of 102.5 degrees F. To help maintain a constant temperature, cows commonly increase water intake and decrease dry matter intake. Decreasing dry matter intake helps lower internal heat production. One of the major factors that contributes to internal heat production is heat generated from the fermentation of fiber, particularly forage fiber. Therefore, part of the solution to eliminating the effects of heat stress is to formulate diets to reduce forage fiber levels. This, in turn, should reduce internal heat production and stimulate dry matter intake.

How can diets be formulated to maximize dry matter intake in heat stressed cows? Dry matter intake of cows is regulated to some extent by gut fill and the forage NDF content of the diet. Forage fiber is required to maintain rumen health and keep the cows chewing. At the same time, forage fiber is one of the least digestible components of the diet. Therefore, excessive levels of forage fiber can lead to excessive gut fill which limits dry matter intake.

As the temperature increases, it is imperative to decrease forage NDF levels as much as possible while still maintaining adequate effective fiber in the diet. This can be achieved by using the highest quality forages available during the summer months. High quality forages are high in energy and low in fiber. This means you should make sure that you have a good supply of corn silage and high quality hay or haylage (>140 RFV) for feeding during the summer. Both of these ingredients are energy dense and low in fiber. Typically diets will be formulated to contain between 20-22% forage NDF. This translates to about 10.5 pounds of forage NDF.

What do you do if the corn silage has run out or the haylage is not of optimum quality? Another method to reduce forage NDF levels in the diets is to simply reduce the amount of forage in the diets and replace it with grain. With low corn prices, it is tempting to reduce forage NDF levels by decreasing forage in the diet and adding more corn. This can work; however, there is a point at which too much corn can be fed. This typically occurs when the nonfiber carbohydrate (NFC) level of the diet exceeds 38-40%. This is not a hard and fast rule. In order to determine when the maximum level of corn is reached, you must read the cows. What are they telling you? Guidelines that can be used to tell if the cows can tolerate more corn are:

1) greater than 50-60% of the cows are lying down and chewing their cuds 3-4 hours after feeding,

2) milk fat test is 3.5% or greater,

3) no more than 20% of the herd has higher protein than fat content (inverted test), and

4) there is at least 8-12% material on the top screen and not more than 55% on the bottom pan of the Penn State shaker box.

If all of these criteria are met, the cows may be telling you that they can tolerate more corn. If the cows fail to meet any of these criteria or if NFC is in excess of 40%, I would be hesitant to provide more corn. In that case, a byproduct fiber source could be used to decrease forage NDF without elevating NFC. Many by-products are high in fiber (soy hulls, wheat midds, cottonseed, etc.). Fiber in byproducts tends to be more digestible than fiber in forages. By removing forage NDF and replacing it with byproduct NDF, we switch from a less digestible fiber source to a more highly digestible fiber source. Improving fiber digestibility in the rumen, should reduce gut fill and improve dry matter intake. During heat stress, the more fermentable byproduct fiber also should reduce internal heat production resulting from fiber digestion. Typically, an investment of less than $.06 for a pound of soy hulls will return $.25 in the form of two pounds of milk. While the investment in byproducts may increase out of pocket feed costs, the return on that investment can be four times greater than that investment.

In times of heat stress it is critical to formulate diets low in forage fiber (NDF) in order to reduce the heat produced by the digestion of these feeds in the rumen. As forage fiber levels are reduced, attention must be paid to milk fat test, cud chewing and particle size of the TMR to ensure that minimum levels of effective fiber are being met to maintain rumen health. In addition to fiber, there are other modifications that can be made to the diet to combat heat stress and we will review those in our next installment.