Drought Stressed Forages

Many areas of Texas have experienced some dry conditions for extended periods this summers. The impact on our forages can be substantial dependent upon the level of moisture shortage, timing of the dry conditions, amount and when nitrogen was applied and forage and soil types. The result can be high nitrate forages, drastically changed forage quality and/or poor yield. This column covers the impact of drought on forage quality and nitrates so that a dairy operation can better survive the recent dry conditions.


The immediate assumption with drought stressed forages is quality in terms of energy and protein, will be reduced. This occurs in severe drought stress but is actually quite rare. More common is moderate drought stress which, while it decreases yield, usually causes improvements in overall quality of the crop. Vineyards have known for years that drought-stressed grapes yield more high quality wine due to increased sugar concentration in the grapes. The same often holds true for alfalfa and corn silage, protein and energy concentrations can increase quite dramatically under dry conditions, provided weed infiltration is not high.

As a result, use wet chemistry initially to analyze stressed forages and consider looking at sugar content as well as routine analysis. The rations should be adjusted and monitored closely to meet the needs of the rumen for effective fiber, protein and energy. Also, introduce the forage as gradually as possible and only feed after it is fully fermented. If these recommendations are followed, cows can perform exceptionally well on drought stressed forages, if they are not heeded, acidosis, displaced abomasum and other problems can result.


Nitrates can accumulate in drought stressed plants such as corn silage, sorghum, sudangrass, oats, orchardgrass, pigweed, lambsquarters, nightshade, ragweed and velvetleaf. High levels of midseason applied nitrogen will also increase nitrates in plants. Nitrates are normal in these feeds, it is just the high level of them that occurs under one or both of these conditions that causes the problem.

Ruminants normally convert nitrates to ammonia in these plants but at high levels it can’t keep up and nitrites accumulate, which are toxic at elevated levels (Table1). If forages are above 1000 parts per million are fed, reproductive problems, abortion, and in acute cases death can occur. Affected animals will have difficult or heavy breathing 3 to 4 hours prior to death, if they recover from the poisoning, abortion may occur days later. In heat stressed herds the animal may go unnoticed until the abortions or death occurs.

Table 1. Nitrate Guide for Ruminating Animals (Dry Matter Basis)

parts per million (ppm)

% Nitrate ion nitrate nitrogen Toxicity

Less than 0.44

0 – 1000

Safe under normal conditions

0.44 – 1.54

1000 – 4000

Avoid feeding to pregnant cows, abortions may result. Feed at less than 25% of diet dry matter.

Greater than 1.54

Greater than 4000

Toxic – Do not feed.

These clues may help you avoid a catastrophe. First, test any suspicious forage prior to feeding. Testing your water supply for nitrates is always a good idea as well. Second, ensile suspect forages if they must be fed, as the forage nitrate levels are reduced by as much as 50% by fermentation. Third, if they must be fed, introduce as gradually as possible. Producers have fed forages with near toxic levels without difficulty when they are introduced gradually, however this is NOT recommended (not feeding it is always better). Fourth, if corn silage is suspected to be high in nitrates, chop high on the stalk. In a test of 28 drought stressed corn samples, Table 2 shows the amount of tested nitrate nitrogen in portions of the corn plant relative to what the whole plant tested. Clearly the bottom 1/3 of the plant is where the nitrates are concentrated. Lastly, as is always the case with problem forages, if they can’t be eliminated, DILUTION is the solution. Dilute out the forage by replacing some of it with low nitrate forage.

Table 2. Nitrate in Corn

Plant Part

Nitrate Nitrogen % of whole plant analysis level (978 ppm)


Less than 7


Less than 2


Greater than 200

Bottom 1/3 of Stalk

Greater than 500

Middle 1/3 of stalk

Approx. 80

Top 1/3 of stalk

Approx. 15

Adapted from the University of Wisconsin


Drought stressed forages do create a number of challenges but they can be minimized through management. Remember to analyze completely and if they must be fed, introduce them gradually and dilute them with “normal” forages as much as possible.

By Tom Earleywine, Ph.D.

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