The authors are with the department of animal science at Iowa State University, Ames.

Cows in early lactation have markedly higher energy requirements, which are not entirely met by energy intake. As a result, cows mobilize fat reserves to compensate for the negative energy balance. This is a normal metabolic mechanism that involves the removal of fatty acids from body reserves to be delivered to the liver for energy production.

Circulating fatty acids in the bloodstream are referred to as nonesterified fatty acids or NEFAs. Hepatic, or liver, metabolism of NEFAs to obtain energy can lead to the formation of acetone, aceto-acetone, and beta-hydroxy butyric acid (BHBA), collectively referred to as ketone bodies.

When the concentration of ketone bodies rises in the bloodstream, cows experience a condition known as ketosis. This condition is not only characterized by a depression in dry matter intake and low milk production, but it is also considered a risk factor for other fresh cow problems such as displaced abomasum and metritis.

Forages are a valuable feed resource that comprise a large proportion of most dairy rations. Normal fermentation for forage preservation is carried out by bacteria that produce lactic acid (LAB) under anaerobic, or oxygen-deprived, conditions.

Accumulation of lactic acid leads to a reduction in pH to a point where no more bacterial activity takes place. This results in effective preservation through “pickling.” If forages are ensiled too wet, for example, less than 30 percent dry matter in corn silage, the environment may not support rapid growth of LAB, and the drop in pH will not be as rapid, allowing growth of bacteria known as Clostridium tyrobutyricum.

In addition to reduction in nutritive quality of the fermented forage, this microorganism produces butyric acid. When C. tyrobutyricum grows and produces butyric acid at or above 0.5 percent of the dry matter, there is an elevated risk of subclinical or clinical ketosis in fresh cows via reduced feed intake, an uptick in the pool of circulating BHBA or a combination of both.

Haylage is different

All fermented forages are susceptible to undesirable fermentation, but alfalfa haylage (or other legumes) is more prone. There are several reasons for this:

1. Legumes have a lower concentration of water soluble carbohydrates or sugars. These sugars serve as the substrate to fuel the bacteria to produce lactic acid, therefore, lower sugar content is associated with less or slower production of lactic acid.

2. Legumes have a higher buffering capacity, which means it takes longer for the acids in the forage to reach a critical concentration to drop pH below 4.5.

3. Since legumes are cut closer to the ground, the risk for soil contamination is greater. Clostridium spores can be present in the soil and come out of dormancy upon ensiling if the conditions are favorable for this microorganism.

The butyric acid blues

Fermented forages with high concentration of butyric acid can have a distinctive foul smell; however, it is very important to have a numeric value to determine the actual concentration of butyric acid. In order to obtain a representative sample, it is important to take samples from multiple points across the front of the forage pile (or bag) and mix them into a single sample. Exercise extreme caution when sampling forages, particularly large silage piles!

Once a sample is collected, sealit in a plastic bag and send it off to a forage lab requesting fermentation profile or VFA (volatile fatty acids) analysis. This test will return the concentration of organic acids, including butyric acid, expressed as a percentage of dry matter.

Regardless of high or low butyric levels in fermented forages, most producers do not have the flexibility of switching to another forage and must feed what they have available. Forages with butyric acid can be fed to lactating dairy cows past their fresh period, lower producing cows as well as dry cows and young stock. Feeding rate should consider a maximum intake of 50 grams of butyric acid per day.

This is the formula used to calculate intake of butyric acid based on lab VFA analysis and ration formulation: (pounds of silage DM in ration times 454 grams per pound)times (percent of silage butyric acid) equals grams of butyric acid intake.

An example for a ration that calls for 15 pounds of corn silage (DM basis) with 0.7 percent butyric acid would result in 47.7 grams of butyric acid per day: (15 pounds corn silage times 454) times 0.007 equals 47.7.

Given the conversion of butyric acid to BHBA, it is highly recommended to avoid feeding this type of forage to close-up and fresh cows to lower the risk of ketosis associated with dietary intake of butyric acid. If unavoidable, talk to your nutritionist to develop a plan of action that can help fresh cows deal with these types of forages.

Promoting greater energy intake and feeding ingredients with high palatability is one strategy. Additionally, mixing other forages to dilute the content of butyric acid is an effective way to lower the risk. Furthermore, as the term volatile fatty acid implies, butyric acid can partially volatilize into the air. Therefore, another strategy is to remove one day’s worth of forage from the storage structure and let it “air out” so that the butyric acid can disperse into the air. In either case, diluting or aerating, a new VFA analysis is required to determine the final concentration of butyric acid and adjust the ration properly.

Avoid clostridial fermentation

Clostridia are naturally present in soil, so soil contamination during harvesting practices or heavy rain will likely introduce clostridium to the storage structure, raising the probability of undesirable fermentation.

Optimal fermentation of forages is driven by the rate of formation and accumulation or organic acids, chiefly lactic acid. One of the most important factors determining the pathway of fermentation is moisture content. Monitor and track dry matter content of forages leading up to the harvest season and plan when to harvest to achieve a desirable dry matter content.

For producers who have the capability of deciding when to harvest corn silage, a rule of thumb is to consider a dry-down rate of 0.5 percentage unit per day. For example, if the target harvest dry matter is 35 percent and the initial sample has 30 percent DM, the harvest date could be set about 10 days after initial sampling (5 percent units difference divided by 0.5 percent per day equals 10 days).

To speed up fermentation and promote a quick drop in pH, include a silage inoculant with LAB that can quickly ferment the soluble sugars into lactic acid. Most inoculants are sold as lyophilized or freeze-dried bacteria that need to be rehydrated and sprayed on to the forage.

As simple as it may seem, proper mixing of water and inoculant is crucial to obtain the best results. Follow manufacturer instructions and use nonchlorinated water to avoid killing the microorganisms in the inoculant.