Talking Agronomy: With Chris Martin - Fine tuning nitrogen planning

Article taken from Arable Farming Magazine

All but the most forward crops will have received some nitrogen by now, but I guess the big question is how much crops will justify in the current economical climate? On the back of last season's high yields in combination with the mild and exceptionally wet December, it is no surprise soil nitrogen supply is lower than average in many fields, encouraging growers to consider increasing nitrogen applications to crops with good potential. The best guide to how accurately you have fertilised your farm historically is to look at your grain nitrogen concentration, which can be calculated by dividing protein content (reported on a 100% dry matter basis) by 5.7. The grain N concentration target is 1.9% for feed wheat and 2.1% for milling wheat. If your grain nitro-gen concentration is less than these figures, it suggests the crop would have responded to more nitrogen and, as a IC, rough guide, you would look to apply an extra 30kg/ha for every 0.1% below target. If your grain nitrogen concentration is at or above these levels, it suggests nitrogen was not your limiting factor to yield. Taking this into account, first wheats are likely to require between 180- 260kg/ha, with the higher end required on nitrogen-hungry soils for crops with good potential and the lower amount required on poorer drought-prone soils.

Second wheats and first wheats following oats are likely to require between 220-260kg/ha. Quality wheats could justify an extra 40-60kg/ha for grain protein. Winter barley is likely to require between 150-180kg/ha depending on soil type and yield potential. For crops with high yield potential, an extra 25kg/ha should be considered for each tonne expected above a standard St/ ha crop, but beware lodging. Nitrogen applications and, in particular, timings for oilseed rape are more crucial. The aim is to build a canopy at the end of flowering with a green area index (GAI) around 3.5. Such a canopy would have the best chance of photosynthesising as efficiently as possible during this key period when seed number is deter-mined.
This canopy would contain about 175kg/ha of N. The skill then is to ensure sufficient nitrogen is available for the remainder of the season for the crop to prolong green area duration (GAD) and maximise seed fill and oil content. To achieve this, backward and forward crops will require different nitrogen regimes. Backward crops should take priority to ensure enough N can be taken up to build optimum canopy by mid-flowering. As a guide, oilseed rape takes up nitrogen at around 3kg/ha day. Some forward crops already had a GA1 in excess of 2 in early March, so already contained more than 100kg/ha nitro-gen.

To build the optimum canopy at flowering they would require less than 75kg/ha N. Taking into account fertiliser inefficiencies, these forward fields would only require up to 125kg/ha of applied nitrogen in order to build the optimum canopy (beware of pigeon damage as when pigeons take the leaves, they also take the nitrogen with them). Having built the optimum canopy, to prolong GAD, additional N (about 30kg/ha for every half tonne expected above 3.5t/ha) should be applied as late as you can get an even spread over the crop (take care in delaying too long if forecast to turn dry) and/or 40kg/ha N should be applied as a foliar application alongside the main sclerotinia spray.