Heat Hurts Dairy Cows' Feet

Help Your Cows Avoid Lameness and Other Ailments if Summer Weather Puts Them Off Feed

When summer heat takes the edge off your dairy herd's appetite, some of them could wind up limping next fall. Making sure your cows eat right when the thermometer tops 20 degrees Celsius can help them ward off subacute ruminal acidosis [SARA], which can lead to other health issues such as lameness.

Cows are much more comfortable at temperatures below 20 degrees than above compared to people. Heat stress starts to set in above that temperature and nutritionists usually see symptoms that can include reduced dry matter intake, lower butterfat percentage and more selective eating patterns in both component-fed and total mixed ration herds. It's been estimated that summer's hottest, most humid days can increase the maintenance energy requirement of dairy cows by as much as 25 per cent.

In addition to the challenges of nutritional management in hot weather, it's also possible that reduced dry matter intake and slug feeding can make SARA worse. It sometimes shows up as lameness in the autumn after a hot summer.

SARA is a disorder of ruminal fermentation characterized by extended periods of depressed ruminal pH below 5.6 to 5.8. Ruminal pH measures the acidity or alkalinity of ruminal fluid. A lower pH means higher acidity. For optimum ruminal fermentation and fibre digestion, ruminal pH should lie between 6.0 and 6.4, although, even in healthy cows, ruminal pH will drop below this level for short periods during the day.

This fluctuation in ruminal pH results from the breakdown of dietary carbohydrates such as starch, particularly from cereal grains like corn, wheat and barley. Grains are high in readily fermentable carbohydrates that are rapidly broken down by ruminal bacteria, leading to production of volatile fatty acids and lactic acid. Under normal feeding conditions, volatile fatty acids are readily absorbed by papillae-small finger-like projections-on the rumen wall. Once absorbed, volatile fatty acids enter the cow's bloodstream and can be used for milk production.

SARA results from excessive volatile fatty acid production that exceeds the ability of the ruminal papillae to absorb them. Volatile fatty acids therefore accumulate in the rumen causing ruminal pH to drop.

In many cases, SARA symptoms are invisible. But daily occurrences of ruminal pH below 6.0 for prolonged periodscan eventually lead to lameness. Low ruminal pH also affects the linings on the walls of the rumen and the small intestine. Reduced feed consumption may intensify the effect of total acid load in the rumen and decrease the ruminal pH further.

A recent University of Guelph study that looked at the effects of heat stress and a high-carbohydrate diet in sheep showed typical responses seen in dairy cattle. The project used a dietary approach developed for dairy cattle that involved feeding a grain supplement at predetermined levels to induce SARA in sheep.

Cattle pant in hot weather and the researchers observed the same reaction in sheep subjected to heat stress. Shallow rapid breathing, or panting, helps dissipate heat by evaporative water loss. Increased respiration rate is also an effective way to exhale carbon dioxide, another mechanism used to adjust the animal's overall pH.

The sheep study results also showed that ruminant animals, such as dairy cattle, try to adjust to conditions of both heat stress and acidosis. They start to use protective mechanisms to remain healthy and comfortable. Countering heat stress includes reducing voluntary activities like walking. They eat less since consuming less feed minimizes heat created by rumen fermentation, digestion, absorption of nutrients and metabolism. The Guelph study also suggested that the animal's panting may also have caused them to consume less feed.

It's been shown that money spent on effective methods to keep a dairy herd's environment cool is almost always a good investment. It can pay off in improved dry matter intake and butterfat percentage along with better overall herd health.

Photos of the ruminal papillae of sheep from the heat stress and subacute ruminal acidosis study at Guelph illustrate the differences in papillae number and length when grain was supplemented, compared with the control diet.

Grain Challenge

Control

The Guelph sheep experiment was the first report on the form and structure of the rumen wall for animals faced with a high-carbohydrate diet under heat-stress conditions. The rumen wall lining not only protects the animal from the rumen's contents but also serves important functions. These include absorption and transportation of nutrients to the bloodstream and some metabolism of volatile fatty acids.

The rumen wall is lined with millions of microscopic, finger-like projections-known as papillae-that do its job. These papillae change shape according to rumen conditions, including pH.

Higher concentrate diets cause these papillae to become longer and there was a trend towards that in the Guelph study. As well, the number of papillae per millimetre of rumen wall was less, as shown in the photos. The differences in length of the papillae and their number did not change the total surface area but the researchers noted that there is a need to better understand the impacts of these changes.

Key points for proper nutritional herd management in hot weather

• Ensure unlimited access to water. Consumption can increase 25 to 30 per cent in hot weather
• Feed more frequently to stimulate feed intake
• Be alert for moldy feed and clean up feed leftovers daily
• Ensure minerals are balanced, particularly sodium, potassium and magnesium. Low ruminal pH can affect magnesium reabsorption.
• Be prepared to increase ration density to offset reduced dry matter intake
• Use rumen additives to promote easier digestion
• Feed more palatable, high-quality forages
• Monitor ration particle size and do not exceed 40 to 42 per cent non-structural carbohydrates in the diet during hot weather

Reference: Odongo, N.E., O. AlZahal, M.I. Lindinger, T.F. Duffield, E.V.Valdes, S.P. Terrell, and B.W. McBride. 2006. Effects of mild heat stress and grain challenge on acid-base balance and rumen tissue histology in lambs. J. Anim. Sci. 84:447-455.

This article first appeared in the Ruminations column of The Milk Producer Magazine, July, 2006.

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