A hidden growth thief in heifer rations

Over the past few months in our veterinary practice in southeast Minnesota, we have noticed a clear pattern as producers begin feeding the 2025 corn silage crop. Feed tests are increasingly showing elevated levels of deoxynivalenol, known as DON or vomitoxin, and zearalenone.

In many herds, the first signs appear as subtle performance changes, including declines in reproduction, milk production and occasional increases in metabolic disease. Because the economic impact is most visible in the milking herd, attention often focuses on the cows. But it raises an important question: Should we also be paying closer attention to our heifers?

Heifers represent the future productivity of every dairy herd, yet their rations often receive less scrutiny than diets formulated for lactating cows. When mycotoxins enter the feed supply, growing animals may be particularly vulnerable.

Why heifers are more sensitive

Zearalenone is especially concerning in young animals because it acts similarly to the hormone estrogen. Even moderate concentrations in feed can interfere with normal reproductive development.

Producers may observe swollen vulvas, vaginal discharge or irregular heat behavior in exposed heifers. In some cases, these symptoms appear early, while in others, reproductive effects may not become apparent until breeding age, when conception rates fall below expectations.

DON affects cattle differently. Rather than directly disrupting reproduction, this toxin primarily influences feed intake and digestive efficiency. Heifers consuming contaminated feed may eat less, grow more slowly or convert feed less efficiently.

Instead of obvious illness, producers may notice heifers falling behind expected growth targets, which can delay breeding age and extend the nonproductive period before animals enter the milking herd. Because heifers weigh less than mature cows, they may also receive a higher toxin dose relative to body weight when consuming the same contaminated feed.

What research shows

Recent research has challenged the belief that cattle are largely protected from mycotoxins through rumen degradation. Studies examining short-term (two days) exposure to Fusarium toxins have demonstrated shifts in rumen microbial populations, including reductions in bacteria responsible for fiber and carbohydrate digestion.

Researchers also observed increased rumination time and chewing activity, suggesting cows attempt to compensate for impaired rumen fermentation. Mild systemic responses, including elevated liver enzymes and changes in body temperature, were also detected.

These findings indicate that even short-term exposure may disrupt rumen function and microbial balance before obvious clinical signs appear.

Understanding risk levels

Nutritionists often recommend conservative thresholds for heifer diets. As a general guideline, DON levels above about 1.5 ppm may begin affecting heifer performance, while zearalenone concentrations above about 0.4 ppm may increase the risk of reproductive effects.

Feeds also rarely contain only one toxin. Corn and corn silage commonly contain multiple Fusarium mycotoxins, and their combined effects may increase the impact on animal health.

When mycotoxin binders may help

If contaminated feed cannot be removed from the ration, mycotoxin binders or detoxifying additives may help reduce risk. Some bind toxins in the digestive tract, while others contain enzymes or microbial components that transform toxins into less harmful compounds.

Traditional clay-based binders are effective against aflatoxin but provide limited protection against Fusarium toxins such as DON. Many modern products combine yeast cell wall components, enzymes and microbial detoxification agents designed to address multiple mycotoxins.

Producers considering a binder should work with their nutritionist or veterinarian to select products supported by research and appropriate for the toxins identified in feed testing.

Feed management still comes first

Managing contaminated feed remains the most effective strategy. Diluting contaminated ingredients with clean feed can reduce toxin concentrations, and higher-risk feeds can be directed to less sensitive animals when possible.

Routine feed testing is particularly important in years when weather conditions favor mold development.

Protecting the herd’s future

Heifer development plays a major role in the productivity of future milking cows. Growth setbacks or reproductive disruptions during development can have lasting effects on herd performance.

Recognizing the risks associated with DON and zearalenone and knowing when strategies such as feed dilution or mycotoxin binders may help and can keep heifers on track to become productive members of the dairy herd.

Portia Seckerson is the owner of Northern Valley Livestock Services in Plainview, MN. Their office of six veterinarians cover Minnesota and Wisconsin. She can be reached at pseckerson@nvls.vet