Factors That Contribute to Subacute Ruminal Acidosis

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Ken Nordlund, DVM — University of Wisconsin-Madison

General Approach to Field Investigations
In field investigations of suspected ruminal acidosis problems, we have found it practical to perform rumenocentesis on two groups of cows. The first group includes periparturient cows, usually between 3 to 20 days-in-milk. A diagnosis of subacute ruminal acidosis in this group will focus the investigation on ration adaptation problems and concentrate overfeeding to early lactation cows. The second group includes cows that are at or near peak milk. A diagnosis of subacute ruminal acidosis in this group will focus the investigation on ration formulation errors and feed delivery problems. By testing representative cows from each of these groups, a herd-based acidosis problem can be localized and identified quite efficiently.

Periparturient Cow Factors
Periparturient cows are at particular risk of ruminal acidosis because of two main reasons. First, the rumen and rumen microbes need to adapt from a very fibrous dry cow diet to the high energy lactation diet in a short period of time. Second, the periparturient cows reduces her intake of feed shortly before and after calving and many dairymen feed excess concentrates during this period of low intake.

Adaptation to high starch rations
Much research work on rumen acidosis has emphasized the adaptation of the rumen microbial population to increased levels of concentrates. Shifts of microbial populations include changes in the concentration of bacteria, as well as changes in the predominant genus and species. Bacteriological studies suggest that about 21 days are required to accomplish these changes and that concentrate levels should be increased at 5 to 7 day intervals throughout the period (Mackie and Gilchrist, 1979).

Dirksen et al. (1985) have emphasized the importance of adaptive changes of the rumen mucosa in the prevention of acidosis. Mean surface area of rumen papillae will increase from 10 mm2 to 60 mm2 when exposed to high concentrate rations, but the process takes from four to six weeks. The larger surface area of adapted papillae is related to increased VFA absorption rates three times greater than rates for unadapted, smaller papillae. If papillae have not elongated, rumen pH can fall, not due to excess production, but because of accumulation in a rumen poorly prepared to remove VFA's.

The take-home message is that the adaptation of microbes and papillae appropriate for a forage-predominant ration to a system capable of utilizing a high-energy lactation ration requires gradual changes over a period of more than three, and perhaps as much as six weeks of time.