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Nitrate toxicity in farm animals PDF Print E-mail

 

Excerpt from 'Paws, Claws & Udder Things - Gribbles Veterinary Pathology'

 

Nitrate toxicity causes significant problems on farms, especially throughout the winter, leading to difficulties deciding how to feed cattle, and how to feed out certain crops. However, stock losses associated with nitrate poisoning can be prevented by analyzing pasture samples prior to grazing.

 

Gribbles Veterinary offers both qualitative and quantitative nitrate analysis using the diphenylamine spot plate method. Qualitative results are typically available the same day that the sample is received at the testing laboratory, and are interpreted as follows:

  • 1% nitrate, generally safe
  • 1-2% nitrate, can cause problems - feed with caution by making sure the cows are not hungry when introduced and are with-drawn within 1 hour
  • 2% nitrate., potentially toxic. - advise not to feed

Quantitative analysis requires the dry matter of the plant material to be determined (water content can vary greatly with weather conditions and plant species) and so results are typically available the day after receipt at the testing laboratory.

 

How plants take up and utilise nitrogen

  • Nitrates (NO3) are absorbed by the roots and travel into the stem
  • Nitrates are converted to nitrite (NO2)
  • Nitrites are converted to ammonia (NH4)
  • Ammonia is then used to produce amino acids and plant proteins.

Under normal growing conditions there is little nitrate accumulation as protein conversion keeps pace with root absorption. However, as growth slows in adverse conditions, roots absorb nitrate faster than the plant can convert it to protein

 

Other factors contributing to increase accumulation of nitrate in plants include:

  • Nitrogen rich soils, either from addition of nitrogenous fertilisers or from fodder crops sown into cultivated old pasture
  • Soils deficient in sulphur and molybdenum
  • Spraying with herbicides
  • Reduced photosynthesis in the winter
It is interesting to note that chopping the plant or ensiling does not cause a decrease in nitrate.

 

Digestion in the ruminant

Ruminants consume large amounts of green feed quickly, then rest and ruminate. The ruminating process involves breakdown by saliva, acid action, microbial digestion and mixing by contractions. This causes a liquid phase which moves on into the omasum and abomasum. Cattle are the most susceptible to nitrate poisoning as they have the biggest rumen of the ruminants and tend to ‘binge’ feed when there is an abundance of food. Other ruminants graze more slowly in comparison.

 

Nitrates in the bloodstream

When nitrate is ingested at low levels it is converted to nitrite which is then converted to harm-less ammonia. When large quantities are ingested too rapidly, however, nitrites are absorbed into the blood stream before they can be converted to ammonia. Nitrite binds with haemoglobin to form methaemoglobin which causes the animal to suffer oxygen deprivation, and affected animals can die within 1-2 hours of moving onto the crop. Rapid veterinary treatment with methylene blue can return haemoglobin to a normal state, although treated pregnant cows will often abort because the foetus has suffered oxygen starvation.

 

Management

By slowly increasing the amount of nitrate rich food consumed, cattle can tolerate higher and higher concentrations as the rumen microflora adapt. With continued exposure, conversion of nitrite to ammonia by the microflora becomes more rapid, allowing the animals to cope with higher nitrate concentrations. Preventing access to large amounts of feed consumed quickly can involve pre-feeding with hay and restricting access to high-nitrate crops. Once the warm weather returns in the spring, pasture/ crop growth increases, resulting in the utilization of stored nitrate and the problem dissipates.

 

Linda Budding - Gribbles Veterinary Pathology

Last Updated on Thursday, 01 August 2013 10:29