Nitrogen use, the endgame

Well the rain just stopped falling in time. Wet ground had just begun to resemble the difficult May of 2006. Cows were leaving holes where they were placing their feet. These holes were filling with water, and the possibility of them being dry again by Christmas was remote.

And the real killer of the lot? The leaves of the grass plant had started to turn yellow on soils that had been saturated once too often.

Again, I had not seen this since May 2006. Not that there was a lot of grass on such fields anyway. This yellowing is purely due to the fact that the plant is totally deficient in nitrogen.

It also probably has a protein content of about 14pc, when the grass plant should have 21-22pc. Feed cows enough of this stuff and a half gallon of milk will disappear quickly. Whether such fields had got a bag of nitrogen or not in early August made no difference. Once soils are saturated all the air pockets fill with water.

Our normally friendly bacteria now become our enemies. They denitrify the nitrogen present, with the result that it disappears into the atmosphere.

And the result? A grass plant that turns yellow due to nitrogen deficiency. Such paddocks should have received at least a bag of CAN last week. They will not grow any grass worth talking about without it.

I would have thought that August was a good month for grass growth, at lease on dry soils in any case. But, dairy farmers coming along to discussion groups with grass covers completed are painting a more mixed picture.

The rainfall has played havoc with nitrogen application, with the result that paddocks that received their nitrogen application in the good weather last week were receiving it two to three weeks later than the planned date.

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Grass growth rates are declining weekly now. The last growth rate figures I received from Curtin's farm in Moorepark showed a growth rate of 66kg/ha/day.

This is still a decent figure. However, such growth rates are at the top end of the scale.

The growth rates figures for the Teagasc farms in Kilmaley, Co Clare, was 36kg of dry matter per hectare. So, some paddocks on the wet soils were hardly growing any grass at all.

What now for nitrogen application? This Friday is the last day that bagged nitrogen can be spread on REPS farms. Farms, outside of REPS have only until September 15.

Nitrogen spread on September 1 will contribute to grass growth for the next six weeks, up to the middle of October.

Any grass growth occurring after that date will be due to background nitrogen (i.e. nitrogen released by the soil only).

Will nitrogen spread today give a better response than nitrogen spread on September 15? Obviously it will. We will have shorter days by September 15 and soil temperature will be on the decline also. However, on non-REPS farms, it makes no sense to spread all the remaining nitrogen this week.

I would wait until the end of the first week. It's a compromise between trying to avail of a better response and going as late as possible.

Some fields will respond better to nitrogen than others. Reseeds, done in the last few years, or fields where two cuts of silage were taken will give a better response than fields that have received nitrogen after each grazing.

While one bag of CAN per acre is plenty in the latter case, up to 40units/ac can be justified in the former.

It is critical to walk the farm once a week at present. Farm covers of 300kg/cow at the moment will make the target of 500kg/cow by mid-September difficult to achieve. Rotation length should be at 30 days by now. If it is still in the mid-twenties, take action before it is too late.


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