DROUGHT STRESSED FORAGES FOR RUMINANTS

 

Limin Kung, Jr., Ph.D.

Livestock Extension Specialist

Professor, Ruminant Nutrition

 

 

Drought conditions stress plants and can lead to the accumulation of several undesirable compounds.  Accumulation of nitrate in forages can be extremely detrimental to animal health.

 
Nitrate Poisoning

 
Many plants can accumulate nitrate under stressful conditions (excessive fertilization or water stress from rain after a drought).  Sunflowers, corn, wheat, barley, rape, bromegrass, and sweet clover are some of the more common plants that can accumulate high levels of nitrates.  Nitrates are converted in the rumen to nitrite and ammonia.  When there is excess nitrate, nitrite accumulates and is absorbed into the blood stream where it binds to hemoglobin and reduces the oxygen carrying capacity of the blood.  Acute poisoning can be observed within 6 hours of forage consumption and is characterized by dark-brown blood, labored breathing, tremors, and weakness.  The following information is primarily aimed at the management of drought stressed corn silage but general concepts are valid for other forages as well.

 
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 Do not graze or feed green chopped forages that have been drought stressed. 

 
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 Ensiling is the best method to manage forages with potentially high levels of nitrates.

 
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 Wait at least 4 to 5 days before chopping drought stressed forage if it is heavily rained on

 
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 For optimal harvest maturity, the degree of drought stress must be assessed.  If ear and kernel development appear normal (+ 80% of normal bushel yields), harvest based on 1) ½ to 2/3 milk line and 2) whole plant dry matter (DM) of 32-35% DM.  In fields where yield of corn is severely reduced, harvest primarily based on whole plant DM%.

 

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Determining whole plant dry matter or moisture is critical because visual assessments can be very inaccurate!  Many plants that look dry contain a significant amount of moisture in the stalk.  Ensiling corn at less than 30% DM may be detrimental due to poor fermentation (clostridia) and excess nutrient runoff.  Harvesting corn too dry (> 40% DM) restricts fermentation, reduces the loss of nitrates, results in forage that is difficult to pack, and can result in excessive spoilage and poor bunk stability.

 
The key issue in drought stressed forages is to harvest material as closely as possible to the optimum moisture or DM content for ensiling.  For corn silage the optimum DM% ranges between 32 and 35%.  If forage is on the dry side, consider, chopping finer to improve packing but remember to balance the TMR for adequate effective fiber.

 

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 In severe instances, because nitrates accumulate in the bottom portion of the plant, raise the cutter bar and leave about 10-12 inches of stalk in the field.  Harvest during the afternoon on a warm and sunny day (nitrate levels tend to be lowest under these conditions).

 

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 Ensiling will decrease nitrate levels by about 50 to 60%.  Allow silage to ferment for at least 3-4 weeks prior to feeding and gradually introduce new silage to animals.

 

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 Use extreme caution around silos because nitrogen oxide gasses that are generated during the first few days of ensiling are lethal to animals and humans!  These gasses tend to accumulate in low areas and are colorless to reddish-brown.  Run the blower for 15 to 20 minutes before entering an upright silo and use caution around vents in silo bags.  Use a respirator before entering a silo.  In severe cases, the gasses will stain forages and other items.  In some instances patches of yellowish silage may be observed.  If these spots of silage have a very low pH (1 - 3) it is possible that nitric acid was formed.

 
· Silage additives:

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Buffered propionic acid-based preservatives: Silage additives based on buffered propionic acid may be an acceptable additive for drought stressed forage especially if the DM% of the whole plant is high: greater than 38 - 40%.  Addition of 2-4 lb./ton of such products per ton of wet forage can improve aerobic stability of the silage and reduce DM losses in the silo and during feedout.  Higher application rates will increase the probability of effectiveness.  Although this may seem costly, such preservation easily pays for itself by preventing drops in intake and milk production that would occur if cows were fed spoiled silage.
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 Heterolactic acid bacteria – Lactobacillus buchneri 40788:  An alternative to buffered acid is to use Lactobacillus buchneri 40788.  This organism safely produces acetic acid, which reduces aerobic spoilage organisms and improves bunk life and aerobic stability.

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 Homolactic acid bacteria (microbial inoculants): If forage is in the normal DM range, consider using a homolactic acid bacteria.  Severely drought stressed corn forage may contain lower numbers of naturally occurring lactic acid bacteria.  Some strains of Lactobacillus plantarum may help with the reduction in nitrates.  Application of the inoculant is best in a liquid form and at the chopper.

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Adding water:  Water can be added back to forage to increase the moisture level, but the amounts needed to have a substantial impact are large.  For example to decrease the dry matter of forage at 50% to 45%, one would have to add 200 lb of water per ton of forage!  In addition, added water can cause run off problems as it is not absorbed efficiently by the forage mass.
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 Sugars:  Drought stressed corn forage usually contains moderately high concentrations of fermentable sugars and because the buffering capacity is low, the addition of molasses or other fermentable substrates is usually not warranted.
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Non protein nitrogen additives: It is best if NPN additives (urea and anhydrous ammonia) are not used on dry, drought stressed forages.

 
· Mechanical processing:

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Mechanical processing of severely drought stressed corn silage has not been well studied.  In our experience, whole plant corn forage harvested at 32-38% DM but with low yields, and corn in the late milk to early dent stage of maturity (before the milkline appears), will have a large proportion of the kernels crushed during normal chopping.  However, processing has been shown to improve the digestibility of the stalk and cob and packing in the silo.  If kernels are extremely milky, back off the rollers to prevent excess runoff in wetter forages.  In drier forages, the rollers may need to be tightened down to yield adequate kernel breakage.

 
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Slow down and set the rollers correctly when using a processor.  It is our observation that in many instances, harvesting (or filling; depending where the processing is done) is occurring at too fast of a speed resulting in poor processing.  Correct processing should result in >90% of the kernels being crushed.

 

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 Drought stressed forage can be harvested as hay but nitrate levels are not reduced by this harvesting procedure.

 

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 After baling or ensiling, all suspect forages should be tested for nutrient composition and nitrates.  Remember to obtain representative samples.  Samples should be kept frozen or refrigerated and taken to the lab as soon as possible.  Drought stress corn silage is usually lower in energy, but higher in protein than normal corn silage and rations should be rebalanced to account for these differences.

 
When feeding high nitrate forages, limit intakes as per Table 1.  Introduce suspect feeds to animals slowly over a 2-3 week period.  Check the nitrates in your water supply too!  Ruminants fed high concentrate diets can tolerate higher levels of nitrates.

 
Table 1.  Safe and toxic nitrate (NO3) levels in feeds.

Nitrate ion,

% DMB

 

Recommendations
0 – 0.44 Safe to feed.
0.45 – 0.88 Usually safe to feed with balanced diet. Limit to 50% of DM intake in pregnant animals.
0.89 – 1.50 Limit intake to 20-25% of DM intake.  Use caution. Do not feed to pregnant animals. 
> 1.50 Toxic!