This year may very well be remembered as the “Year of the Extremes,” at least weather-wise.  While we in parts of the South and Southwest are burning up with record heat and drought, other areas of the country are wringing wet. Neither of these conditions is ideal for cows and forages. 

While you understand very well that stressful situations negatively affect the immune system of the cow and make her more vulnerable to pathogens, you need to also remember that you’ll tend to see more toxicoses from forages in stressful growing conditions.

Poisonous results
A toxin is another name for a poison. Most toxic responses (also known as toxicosis) increase in severity as the amount of toxicant increases. Plant toxins are secondary products of plant metabolism, meaning that they are not essential to plant growth or reproduction. Factors favoring plant-related toxicosis include adverse climatic conditions (like drought), agricultural practices and animal-management practices.   

In drought situations, it is important to recognize that the agricultural practice of nitrogen application becomes a critical factor of which to be aware. For example, nitrates accumulate in the lower part of corn when stress reduces yield to less than the supplied nitrogen fertility level. Ensiling will reduce the level of nitrates by roughly 50 percent. Nitrate levels of less than 1,000 parts per million are considered safe under all conditions. 

Animal-management practices may place animals at risk in their environment, too.  Although cows are fairly discriminate eaters, they will eat toxic plants even though most are relatively unpalatable. Nutrient deficiencies may induce abnormal appetites. Overgrazing can cause an increase in weeds (some of which may be toxic) and reduced amounts of forage leading cows to consume toxic plants. Placement of thirsty or hungry animals in an unaccustomed location can lead to the consumption of toxic plants, as well. 

Other sources
Mycotoxins are also factors to watch. Mycotoxins are secondary metabolites of fungi that are recognized as toxic to other life forms. They are divided into two categories: field and storage. 

Field fungi grow under conditions occurring prior to harvest. Fusarium is an important toxin-producer in this category. Field fungi usually require relative humidity about 80 percent and a grain moisture above 22 percent. Field fungi invade seeds on field plants (corn kernels), but do not persist under normal storage conditions.

Conversely, storage fungi do not invade intact grain prior to harvest. Ideally, they blossom at a relative humidly of 70 percent and grain moisture content of 14 to 24 percent. Any condition that affects the seed coat, like insects, drought stress, mechanical harvest or hot air drying, may enhance fungal invasion. Two common genera in this category are Aspergillus and Penicillium. 

Conduct an investigation
If you suspect an agent of toxicosis may be present in your herd, help your veterinarian to gather the important information needed for a diagnosis. The history of exposure and presence of a toxicant in the environment are essential to know. Note any animal or management adjustments like changes in animal location, food sources, recent chemical application and other events. 

Finding a toxicant in the environment or even knowing about consumption does not close the case, but suggests further investigation. Make note of clinical signs and the organ system(s) affected. Clinical signs alone are not significant as many signs of poisoning (vomiting, seizures) can also be caused by infectious diseases or metabolic disorders.

Your veterinarian can help guide you on sample collection. Animal, environmental and necropsy samples are necessary for a definitive diagnosis. 

In conclusion, be aware of the effects of extreme weather not only on cattle, but the feed that they consume.  

Angela M. Daniels is a veterinarian with Circle H Headquarters LLC, a dairy and swine veterinary practice, food safety laboratory and DHIA milk-testing and contract research organization in Dalhart, Texas.