Wednesday, 5 August 2020
The extreme drought conditions in most of the North Island and top of the South, wet conditions in Southland and the West Coast coupled with farmers holding more stock over autumn and winter (due to Covid-19) has created huge pressure on pasture supply. Supplementary feeds are in very short supply, and expensive, and winter forage crop growth has also been poorer than normal.
Unless we have an extremely mild winter, there will still be considerable pressure on pasture supply moving into spring. Tactical use of nitrogen (N) fertiliser in late winter/ early spring, targeted so the extra grass grown satisfies a specific animal feed requirement, will be a cost-effective way of providing some much-needed feed.
Given the vibrant microbiology in our grazed pasture soils, trial work shows little difference in speed and size of the N response to the type of solid N fertiliser, e.g. urea, DAP or ammonium sulphate, provided other nutrients are not limiting. This is because all forms of N are transformed by the microbes to nitrate within days to a couple of weeks. However, in cool and wet conditions there are other limiting factors that need to be understood when picking the right N fertiliser.
In late winter/early spring, after winter drainage events have leached much of the sulphate sulphur (S) present in soil, there may be a temporary lack of available S for the pasture as it responds to increasing daylength and temperature. Additionally, the crucial soil bacteria that mineralise S from soil organic matter for your pasture may not be fully active due to cold and wet conditions. Sometimes there will be a short-term S deficiency, which will limit the response of urea N, while ammonium sulphate applied at the same rate of N may give a bigger and longer response due to the added available sulphate sulphur.
While we have no way of predicting where or when these limitations will occur for cold and wet late winter/early spring conditions, a product such as Ammo 31 or 36 is ideal as it combines both urea and ammonium sulphate and should be effective at minimising this risk. The reason for the better response to an Ammo product compared to urea under these conditions is likely due to two of the essential protein- forming amino acids, cysteine and methionine, which require sulphur as part of their structure. If the pasture plants can’t form enough of these amino acids, then the N response will be less.
You may well ask that if an N fertiliser contains some nitrate, e.g. calcium ammonium nitrate or ammonium sulphate nitrate, will this work better in cold conditions? Trials have shown that even at soil temperatures between 2-6°C for three to four weeks after application, the extra response is barely measurable and not large enough to justify the much higher cost per kilogram of N in these products when applied to pastures.