Wet spring delays fertiliser applications
THE wet start to spring has already delayed the timing of fertiliser applications for many farmers. It’s therefore important to consider the implications of when nitrogen is applied and optimal cutting dates for silage.
If fertiliser is applied too close to the cutting date, then problems can occur in the fermentation process, leading to poor quality silage. Most significantly, the sugar content can be reduced due to the rapid growth of grass and grass will have elevated levels of nitrates. The target level for grass at cutting is below 0.1% (1000 mg/kg fresh weight) nitrate-N.
Nitrates in silage cannot be used as a nitrogen source by the animal. Nitrates are converted to ammonia in the silage which increases the pH, and alongside low sugar content, results in a slower, poorer fermentation.
This will aect palatability and therefore reduce intakes and animal performance. High levels of nitrate may be converted to nitrite in the rumen which is toxic to animals.
One method to estimate when all nitrogen has been utilised by the crop, is to work out the nitrogen uptake per day from the date of application.
Table 1 assumes an N uptake of 2.5kg/ha (2 units/ acre) per day from the date of application. This is assuming active uptake, therefore a safety of + 7 days could be applied if conditions are unfavourable. See table.
From the table you can see at an application rate of 60kg N/ha all the nitrogen is utilised four weeks post application. In comparison with a dressing of 120kg N/ha all the nitrogen is utilised seven weeks post application (assuming optimal conditions).
Another way to ensure nitrate-N levels of the grass is below the target level of 0.1% is by using test strips for nitrates, which can be purchased online and the test performed on farm or by sending grass samples to a lab for analysis. SAC Consulting in partnership with Eurofins is one lab that can oer the test.
Nitrogen is easily lost to the air so if sending samples to a lab it is worth bearing this in mind.
Send samples early in the week and ensure they
are packaged in a tightly sealed container or bag with as much air excluded as possible.
If the grass is above 0.1% there is some risk to silage
quality (dependant on the sugar and dry matter content of the grass), therefore it is best to delay cutting and retest a fresh grass sample in 3-5 days.