Conditions might result in producers purchasing hay, perhaps from long distances and previously unknown sources. While recommended every year, obtaining a forage nutrient analysis on hay becomes even more important in this situation. Remember: It’s hard to manage if you don’t measure first. A forage nutrient analysis is a critical step in determining least‐cost rations for wintering beef cows. This month’s newsletter will cover how to interpret forage nutrient analyses for beef cattle. Below is an example nutrient analysis for a grass hay, including minerals. Definitions of important terms are found on the next page, along with energy and protein requirements for a 1400‐ pound cow.
When comparing the cow requirements to the forage analysis, the hay would meet requirements pre‐calving, but would not meet requirements for early lactation. Note that the listed requirements are for mature cows, not replacement heifers or first‐calf heifers. Younger cows will have higher requirements for both energy and protein.
Producers purchasing hay containing forages prone to nitrate accumulation should also request a nitrate test. Annual small grain crops, corn, sorghum, sudangrass, and many common weeds are prone to accumulate nitrate in drought conditions. Nitrate uptake is normal part of plant metabolism. Nitrate is converted to nitrite, which is then converted to ammonia for protein synthesis in the plant. These conversions happen in the leaves of the plant, so any conditions negatively impacting leaves (drought, hail damage, etc.) may result in nitrate accumulation in these forage types. Nitrate concentrations are highest in the stem or the stalk, especially the lower portion.
The nitrate conversion pathway is exactly the same in the rumen as in plants; however, high nitrate concentrations overwhelm the conversion pathway from nitrite to ammonia and nitrite accumulates. As nitrite enters the bloodstream, it competes with oxygen for red blood cells. Nitrite converts hemoglobin to methemoglobin, which is incapable of oxygen transport. Chronic nitrate toxicity can result in reduced appetite and milk production, unthrifty appearance, poor gain performance, or abortion. Acute nitrate toxicity generally results in death from lack of oxygen. Symptoms include accelerated pulse rate, labored breathing, muscle tremors, cyanosis (blue mucus membranes), and death.
The table below provides a summary for interpretation of nitrate concentrations in forages. Note that these numbers are more conservative than those you might receive from a testing laboratory or in publications from other states. Also note that it is critical to match the method of nitrate measurement on the analysis to the correct column in the table. Nitrate‐nitrogen (NO3‐N) is a different measurement than nitrate (NO3). If the nitrate concentration is reported in percent, move the decimal 4 places to the right to convert to parts per million. For example, 0.18% NO3 is the same as 1800 ppm NO3.