Nitrate poisoning in cattle is caused by the consumption of feed or water containing high levels of nitrate nitrogen. Under most circumstances, forage crops do not contain levels of nitrates high enough to be toxic. When this does occur, however, it is a serious problem, often resulting in the death of many animals.
Factors Influencing Nitrate Accumulation
Nitrates accumulate during periods of stress in plant growth, such as low soil moisture and high temperatures or low humidity. The accumulation of nitrates is often favored by heavy nitrogen fertilization rates, particularly if the fertilization is timed to coincide with the onset of a drought period.
Under conditions of adequate moisture availability, hay produced in Alabama should pose no threat of nitrate poisoning. Hay cut during or immediately following a drought period may have potential for nitrate accumulation. This is particularly true in cases when applications of nitrogen were made to forage crops, but little growth was made before hay harvest because of lack of moisture. It is important to note that nitrates degrade little with drying or ensiling of forage and, consequently, may cause toxicity months after harvest.
Some commonly grown forage crops in Alabama that are known for their potential for accumulation of toxic levels of nitrates are sorghum, corn, sudangrass, sorghum-sudan hybrids, soybeans, fescue, pearlmillet, and bermudagrass.
In the Southeast, problems are most likely to be encountered with warm-season grass crops such as bermudagrass or a summer annual forage. This is primarily because these species are most likely to receive high levels of nitrogen fertilization that favor nitrate accumulation in plant tissues.
It is also possible for weeds to accumulate toxic levels of nitrates. Therefore, weedy hay could pose more of a nitrate toxicity threat than weed-free hay. Some weeds that are known to accumulate nitrates are pigweeds, smartweed, lambsquarter, Canadian thistle, ragweeds, nightshades, and stinging nettle.
Pigweed is associated with nitrate poisoning more frequently than an other weed commonly found.