Sub clover is a key driver of any grazing system, it improves animal performance and adds nitrogen to the soil that is used by grasses or in a subsequent cropping phase.
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The amount of nitrogen ‘fixed’ by the sub clover can vary considerably. While figures of 25kg of N per tonne of dry matter are commonly quoted for newly inoculated sub clover pastures, this amount declines over time.
Soil acidity, deficiencies in trace elements such as molybdenum and the natural replacement of the more efficient inoculated rhizobia with less efficient native strains means the amount of N fixed, may not be as high as we think.
So what can be done to maximise the amount of sub clover we grow?
Sub clover has an annual lifecycle and to persist it needs to set enough seed each year to produce high numbers of seedlings the following autumn.
Sub clover has three mechanisms to guard against false breaks and ensure persistence – embryo dormancy, temperature dormancy and hard seed.
All sub clover cultivars develop hard seed or an impermeable seed coat to water, but they vary in the rate the hard seed coat breaks down. The rate of breakdown depends on firstly the cultivar and secondly, on the day and night fluctuation in temperature.
Excessive trash or dry material left on the soil surface over the summer-autumn period acts as insulation and prevents the seed from being exposed to temperature fluctuations that cause the seed coat to crack.
The greater the fluctuation in temperature, the more rapid the breakdown.
There is an obvious compromise on how much dry materials to leave behind. Too much dry matter insulates the seed, whereas too little, while breaking hard seed, can expose the soil to wind and erosion when it rains.
Recent work conducted by Southern Farming Systems (SFS) showed if there was excessive dry matter over summer only seven sub-clover plants per m2 germinated.
At the ideal level (of approximately 1000 kg/ha), germination was 182 plants per m2. Where all the trash had been removed, essentially overgrazing, the sub clover germination was similar to the ideal.
The difference in germination led to huge difference in clover content of the pasture later in the year.
Strategies for dry matter removal generally involve short rotations of grazing at high stocking density.
If this is not possible across the whole farm, this summer prioritise a couple of paddocks and try getting the trash management right. You’ll probably end up having a good clover year on those paddocks.