March 2019

Soil and Pasture Nitrate after a dry spell

Farmers are aware of a flush of grass after a dry spell but reasons for this rapid growth are often over looked.  Pasture growth stops during drought, but mineralization of soil Organic Matter continues to release Nitrate-N into the soil.  The effect is that soil Nitrate-N levels accumulate, and pasture utilization of soil N has slowed or stopped depending on the severity of the drought.
When drought-breaking rain finally arrives, growth of surviving pasture will resume utilizing the accumulation of soil Nitrate-N.  Post-drought pasture grass growth is typically lush with a high Nitrate-N content, very high Protein, high Potassium and low Calcium compared to dairy cow dietary requirements.
The effect of summer drought that is severe enough to cause the grass to ‘brown-off’ and go dormant is that root carbohydrate reserves are depleted, and root mass is reduced.  Ideal ‘post-drought’ management of ryegrass should allow pastures to grow to ‘2.5 to 3 leaf stage’, or about 30 days growth before grazing.  The effect of this grazing interval is to allow the pasture grass to start to replenish root reserves, minimize the risk of Nitrate-N toxicity to livestock and to provide appropriate Protein, Calcium and Potassium for good feed conversion efficiency.
Farmers are short of feed for livestock, so when pasture growth resumes, the tendency is to feed livestock on fresh re-growth.  Pasture that is grazed before the ‘2 leaf stage’ will lack vigor and ultimately be less productive.  Early grazing of ‘post-drought’ pasture growth will significantly increase the risk of Nitrate-N toxicity, and excessively high Protein and Potassium in addition to low Calcium will increase the risk of metabolic stress.
Where there is no alternative to early grazing of recovering pasture, feeding of supplements that have a high carbohydrate and/or fiber content will improve feed conversion efficiency and reduce the risk of metabolic stress to stock.  Submit samples of pasture immediately prior to the first grazing post-drought for a Nitrate-N test. This will provide useful information for livestock grazing management in addition to indicating when pasture grass may start to become responsive to N fertiliser.
Application of N fertiliser to pasture recovering from drought may not produce a growth response for about 2 weeks after substantial rainfall because of accumulated soil Nitrate.
Early application of N fertiliser could increase the risk of Nitrate-N poisoning.