Farm Ireland

Monday 22 January 2018

Don't let the rain ruin grass growth

Dr Mary Kinston

The depressing summer weather conditions have continued. As a consequence, many farmers, especially those on the wetter farms, are feeding silage and supplement on a daily basis.

Cows have been housed on numerous farms either by night, with an on/off grazing regime, or full-time, where grazing conditions and grass availability remain poor. It seems to be Murphy's law at this stage that lower milk prices are coinciding with higher costs, with this year's profitability severely challenged by this dull and wet summer.

However, although the inclement weather has continued, a decrease in rainfall from deluges of greater than 40ml per week to closer to 25ml per week has seen a small improvement in ground conditions on our farm in Co Kerry. This development has been matched on many others, too.

A walk of the farm revealed that paddock conditions ranged from reasonably firm to soft with wet patches. And some marginal ground was still waterlogged. At this stage, our grass looks 'hungry', with a yellowish-green colour evident throughout the majority of the paddocks.

But the paddocks that haven't received fertiliser in more than four weeks look most alarming. The grass here is slightly brown and growing very poorly. The slight ease in rainfall has increased the opportunity to spread fertiliser. But the question is: where, when, what and how much fertiliser to spread?

Here are a few interesting facts about nitrogen and wet weather to help you consider the options:

nNitrate is essential for plant growth and is perceived mainly as a chemical fertiliser. However, much of the nitrate found in soil is produced by microbes that break down plant and nitrogen-containing residues.

nThe activity of these microbes is affected by soil temperature and moisture.

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nWeather has a marked effect on the amount of nitrogen required, as the amount of rainfall determines how much fertiliser a plant needs. In ideal weather conditions, soil supplies a considerable amount of nitrogen which is readily absorbed by the plant's roots and decreases N fertiliser requirements.

nThe greater the amount of water in the soil, the slower its temperature will rise. This is partly due to the heat required to raise the temperature of the extra water, but mainly because of the heat required to evaporate extra moisture. This is why poorly-drained clay soils are colder than free-draining soils, in which greater amounts of water evaporate.

nExcessive rainfall also increases fertiliser requirements by leaching nitrogen from the soil and increasing losses through denitrification. This is the conversion of nitrate to gaseous nitrogen, which is then lost to the atmosphere.

nWater moves vertically through sandy soils increasing leaching losses. In more structural soils, especially clays, drainage occurs mainly through fissures and channels and by-passes much of the soil. This reduces leaching -- including the loss of nitrogen and potassium -- but increases the loss of immobile nutrients such as phosphorus.

nWhen soil conditions are poor, extra nitrogen is often needed for growth. As a result, grass growing in poor conditions often shows signs of nitrogen deficiency.

In summary, paddocks can turn yellow in colour if they are deficient in nitrogen. Here, wet and waterlogged conditions increase the loss of any available nitrogen through leaching and denitrification. Availability of organic nitrate is also limited due to adverse conditions for soil microbes. Clay soils suffer worst of all as they are cooler.

Trace elements should also be considered. Magnesium deficiency occurs in adverse soil conditions. Magnesium is important for cow health and discoloured grass could indicate low levels in the soil.

Nitrogen -- When and Where?

If a paddock's soil is saturated, do not apply nitrogen. It's important to wait until wet conditions have improved to reduce nitrogen losses. Also, check the weather forecast before spreading. Avoid spreading fertiliser where heavy rain is forecast within 48 hours, although this is easier said than done.

What fertiliser?

The efficiency of N is limited by other nutrient deficiencies -- the main nutrients being phosphorus (P), potassium (K) and sulphur (S) -- especially where high rates of nitrogen are used, so consider your soil test values and what compound fertiliser you have spread this year. A reduction in efficiency can be limited with regular P, K and S maintenance dressings. Note that high rainfall promotes the efficiency of urea by moving it below the soil surface.

How much?

Soils have a finite capacity to store N in its organic matter; therefore higher dressings of N will result in increased losses. Consequently, apply in split dressings of no more than 40 units N/acre under grazing, with most farmers applying 20-30 units per application.

As the soil is rather deficient, higher applications of 30-40 units may boost growth if ground conditions are suitable.

Dr Mary Kinston is an agricultural consultant and discussion group facilitator and can be contact ed at

Indo Farming