With the growing season rainfall (GSR) for Birchip currently sitting at a decile one, a group of young farmers gathered recently to discuss how they might take the risk and agony out of nitrogen (N) decisions.
In any season, deciding when and how much N to apply can be difficult. The table below illustrates why timing N applications can be so difficult. In Swan Hill, for example, there is, on average, only nine rainfall events during the year above 10mm. If you subtract half the days which fall outside the growing season, and a few more days where the events fall outside the N spreading window, it leaves only about three opportunities to top-dress.
Table One: Mean days of rainfall above 10 mm and above 20 mm for a whole year
Table One: Mean days of rainfall above 10 mm and above 20 mm for a whole year
|
Location |
Rain |
Mean days |
Period |
|
Ouyen |
≥ 10mm |
9.1 |
1911-2012 |
|
≥ 25mm |
1.6 |
||
|
Swan Hill |
≥ 10mm |
9.4 |
1996-2012 |
|
≥ 25mm |
0 |
||
|
Wycheproof |
≥ 10mm |
10.7 |
1883-2008 |
|
≥ 25mm |
2.1 |
||
|
Longerenong |
≥ 10mm |
8.5 |
1863-2012 |
|
≥ 25mm |
1.4 |
When trying to determine a crop’s nutritional requirement, a good starting point is its yield potential. This can be calculated using the French and Shultz equation, and is relatively easy given some practice. Table two lists some potential yields for 2012 calculated with different possible rainfall and stored moisture in Sea Lake.
Table two: Potential rainfall and yield for different deciles at Sea Lake*
|
Sea Lake |
Potential rainfall Apr-Oct |
Potential yield (t/ha) with zero stored moisture |
Potential yield (t/ha) with 40mm stored moisture |
|
Decile1 |
129 |
0.3 |
1.6 |
|
Decile 3 |
183 |
1.3 |
2.0 |
|
Decile 5 |
214 |
1.8 |
2.5 |
|
Decile 8 |
277 |
3.0 |
3.7 |
*Using French and Shultz equation with a water use efficiency of 18.
The next step is the tricky bit – confirming what to actually put on. Before making this decision growers should consider:
· How much N is currently in the soil?
· The quality of grain being aimed for?
· What sort of season is expected? It needs to rain quite a bit from now until the spring to hit an average GSR.
· What growth stage top-dressing should occur?
· When urea needs to be ordered?
· The return on investment. The cost of the product and its application (spreading) needs to be considered in conjunction with grain prices (currently wheat is sitting just above $220 at Birchip for APW). If the price of nitrogen is $650/t ($1.40/kg) the cost to top-dress a paddock at a rate of 40kg/ha will be $56/ha.
A worksheet detailing how to calculate, yield potential and a nitrogen budget was distributed and explained to the young farmers at their meeting.
