Farming Magazine - May, 2008


Dairy Nutrition: Managing the Rumen

By John S. Hibma

This past February, I attended the Southwest Nutrition and Management Conference. Several of the talks focused on the importance of maintaining a consistent feeding schedule in order to avoid large fluctuations in rumen acidity during the course of the day. High-producing cows with high intakes of starchy concentrates are the most prone to a condition called subacute rumen acidosis, or SARA. We may also refer to this as subclinical rumen acidosis. Even with well balanced, well managed diets or total mixed rations (TMR), and the absence of slug-feeding, acid levels in the rumen can fluctuate to dangerously low levels, disrupting microbial health and feed fermentation.

We measure acidity with a scale called pH, which essentially tells us how many hydrogen ions are present. The pH scale runs from 1 to 14, with the value of 7 being neutral, neither acidic nor alkaline. The lower the number, the more acidic the environment. The higher the number, the more alkaline. Because the actual pH measurement of rumen fluids can’t be done on the dairy farm, the best way dairy farmers and managers can tell if cows may be experiencing SARA is by observation of manure, milk butterfat test and the use of feed ration modeling software.

Ruminant scientists generally accept rumen pH from 6.0 to 6.5—mildly acidic—as being optimal for the healthy function and proliferation of rumen bacteria—a fairly tight range. Anything lower than that will slow down rumen fermentation and, according to resear-chers, a cow is considered to have SARA when rumen pH drops to 5.5 or lower. Research shows that rumen pH can fluctuate significantly over a 24-hour period when a cow is fed twice per day—and that’s how most dairy farmers feed their cows. The chart shows a rumen pH study for a cow fed cracked corn and alfalfa haylage twice daily (12-hour interval). The pH ranged from 6.6 to 5.4 over 24 hours, dropping to 5.5 numerous times during a 10-hour period. It’s also interesting to note that the pH for this cow varied, numerous times, from 3 to 6 points in just an hour’s interval.

Rumen pH will drop when excessive amounts of rapidly fermentable carbohydrates such as starches and sugars are fed. The pH will also drop if inadequate levels of fiber are offered in a diet, or a combination of both of these. So, dairy farmers and nutritionists have focused heavily in recent years to formulate feed rations that have properly balanced carbohydrate fractions.

More recent research has confirmed that the disruption or interruption of a cow’s diet can have even more impact on pH fluctuations than just the diet formulation itself. On many dairies it’s quite common for cows to go without feed for several hours or more due to management choices, such as milking schedules. Periods of very high rumen pH (an alkaline condition) due to feed deprivation may occur, having detrimental effects on those bacteria that are sensitive to a high pH. The disruption of feeding schedules ultimately results in a real see-saw in the rumen environment, often taking days to stabilize and reestablish the bacterial population. Dairy farmers and their managers may not see or believe that there is any significant effects on milk production, but many is the time that otherwise unexplained drops in milk production or milk components can be traced back to a feed management issue.

Normally, the cow does a good job of keeping her rumen buffered through saliva production and her ability to compensate quickly with absorption of excessive rumen acid across the rumen wall. But, poor diet and feeding management on the part of the dairy farmer can quickly overwhelm the cow’s ability to compensate.

Therefore, to compensate for occasional poor management due to late feed deliveries, oversleeping, bad weather, holidays or whatever, we’ve been including dietary buffers in dairy feed rations for many years. Sodium bicarbonate (SB) has been the most widely used buffer and extensive studies have been conducted on its use.

Cows in early and mid-lactation have the most to gain with the inclusion of SB in the diet since they have the highest energy needs, requiring high-starch diets. Studies have also shown that corn silage diets will nearly always benefit from the inclusion of SB. Research suggests there’s little or no benefit with supplementing SB in high hay and haylage diets since those forages tend to supply a large amount of buffering capacity in and of themselves. This would include pasture-based diets. However, the question always remains as to whether high hay or haylage diets can supply enough energy to a high-producing milk cow. Once total dietary starch is near 25 percent of the ration dry matter, SB should be added. Typically, a quarter to half a pound of SB will provide adequate buffering. Excessively wet rations will need the half a pound. Inclusion of SB over 1 percent of the diet dry matter has been determined to be ineffective. In other words, a properly formulated diet for a cow consuming 50 pounds of dry matter will require only half a pound of SB to accomplish adequate buffering—any more will be a waste of money.

The inclusion of SB in the diet will often enable a cow to increase her feed dry matter consumption. Often, though, the increase will not result in more milk production, but, rather, milk butterfat percentage and yield will increase. Responses can vary due to individual cow responses and also the management capabilities, or lack thereof, of the dairy farmer.

Nutrition and management on a dairy farm tend to go hand-in-hand. Because proper nutrition is such a large, influential factor in milk production, feeding cows and the daily management of a feeding program should be a top priority—right up there with milking and reproduction management. Even if you’re the best dairy manager in the world with expertly balanced diets intended towards making the highest levels of milk production, the inclusion of a dietary buffer in your cows’ diets is pretty much a necessity.

The author is a dairy nutritional consultant and works for Central Connecticut Farmer’s Cooperative in Manchester, Conn.