Wednesday, 1 April 2020

Nitrogen key weapon in drought recovery


Yet another dry summer has hit many parts of New Zealand and while we have not had enough rain in the drier areas to alleviate the effects, the climate has moved from a hot dry summer to autumnal conditions reasonably rapidly.

Dr Ants Roberts, Ravensdown’s Chief Scientific Officer, says after the first significant rain, a relatively quick and cost-effective option to increase pasture production is to apply nitrogen (N) fertiliser.  This N fertiliser should be coated with a urease inhibitor e.g. N-Protect, unless you can guarantee that at least 10mm of rain will fall within eight hours of application. One pass of a pivot irrigator is unlikely to deliver that amount of moisture and by applying less than 10mm within eight hours, volatilisation losses can be as bad as having no moisture at all.

 “It is preferable to wait for some regrowth but in post-drought conditions with days getting shorter and cooling temperatures this is unlikely to be practical.  Act early - farmers should apply N fertiliser as soon as possible after significant rain has fallen, while soil temperatures remain above 7-8oC. Sufficient rain means that there is enough moisture to keep pastures growing for at least 4-8 weeks after N application” Dr Roberts says. “This will allow you to get the maximum benefit from the N application in terms of pasture or crop response.”

Top autumn drought recovery tips

  • Use N-Protect if you can’t guarantee 10mm rain within 8 hours
  • Act early with cooling temperatures
  • Renewed pastures, high fertility soils and modern cultivars will recover fastest
  • Spell paddocks for 3-4 weeks before grazing
  • Use lower rates of N over larger areas
  • Target biopesticide application to low pasture yield areas for grass grub mitigation.

Professor Cory Matthew from Massey University also demonstrated a further benefit to autumn applied N. Autumn N allowed the grass plants to produce new tillers, which are severely depleted during drought, and this sets the pasture up for the coming spring.

Renewed pastures as well as paddocks with high fertility soils and modern cultivars, will have the capacity to recover fastest from drought conditions, producing the greatest dry matter responses. With older pastures, research indicates that they will still give cost-effective responses to N fertiliser.  Ideally, it is best to apply N to those paddocks with some cover (i.e. 1,500kg DM/ha or better).

Dr Roberts says it’s preferable to spell paddocks for as long as possible (3- 4 weeks) before grazing as this gives the pasture time to express the maximum response possible, although set-stocked paddocks will also give N responses. 

“As the rate of N applied increases, the efficiency of response (the kg DM grown per kg N applied) decreases and individual applications of fertiliser N should ideally not exceed 60kg N/ha in high potential growth rate conditions and 40kg N/ha under poorer conditions. Remember that N fertiliser is a growth multiplier and so the faster the pasture is growing, both during and following N application, the better the response.”

In general, it is best to use lower rates on larger areas rather than the reverse. Farmers should budget on a 6-8kg DM/kg response to N applied in autumn and 8-10kg DM/kg N applied in late winter/early spring at these rates.

Provided the fertiliser history has been good in the past and soil fertility status is reasonable, then foregoing the spring maintenance fertiliser in favour of just N is an option if finances are tight. It might be possible to ‘catch up’ with maintenance fertiliser later.

Another way to ensure your pasture recovery has its best shot is to ensure grass grub infestations in late autumn are kept to a minimum by targeting application of biopesticides to low pasture yield areas.

“Some recent research we conducted with AgResearch showed that this approach on a typical 225ha Canterbury dairy farm (system 4) could gain you an extra 10 tonne of dry matter in autumn,” Roberts says. “For a lot of the country right now every little bit helps as we set ourselves up for winter.”