Advice on using fertiliser in warm weather
With the current dry spell looking likely to continue for further days or weeks, grass growth will come to a halt on more farms than are currently affected.
Presently soil moisture deficits (SMD) range from some 90mm in the south east to some 60mm in the north west. Once SMD is over 50mm, grass growth becomes restricted, and at more than 75mm, drought conditions are occurring and perennial ryegrass will go dormant.
Large quantities of rainfall will be required to fill these soil moisture deficits in the soil, with 25mm being equivalent to an inch of rain.
Fertiliser and slurry applications will show no apparent benefit for grass growth until it rains and adequate soil moisture returns.
However, much of the fertiliser applied in the last four to six weeks will remain in the soil. These will be re-activated once adequate rainfall occurs and growth conditions improve.
It is best to hold off on any further N fertiliser applications until temperatures decrease (such as with dew at night) or prior to when rain is forecast on fields that were not fertilised in the last month.
On fields that received a N fertiliser application after the last grazing rounds (such as where residual N is present in the soil) hold off on further N, fertiliser applications until after it rains, as the residual N in the soil will likely be sufficient to kick off grass growth.
In drier soil conditions, fertiliser containing Ammonium Nitrate and Calcium Ammonium Nitrate as their N source are safer to spread and have lower risk of volatilisation loss to the atmosphere.