Clover and nitrogen and the costs

By Bernie Mitchell

Farmers often question the benefits of an all grass plus fertiliser N system compared to that of a more natural clover based system.

To answer some of these queries Doug Edmeades, of the Fertiliser Review, had this to say:

The N inputs into a clover-based pasture are self regulating. The more stress that is applied to the clover the less N is added to the soil (via clover symbiotic N fixation) and the more the soil becomes N deficient - but the more N deficient the soil the more the clover flourishes (it does not need soil N, unlike grasses, to grow). The more it flourishes the more N is added to the soils. This is why the clover content of pasture changes on a cyclical pattern.

The size of the clover pest population, and hence the damage from pasture pests, is also cyclical. They build up, do their thing, and when the ‘food’ is gone, move on. During their active-damage stage they will reduce the amount of clover growth and hence N inputs of atmospheric N. The soil will become more N deficient and hence, when the flea flees, the clover will flourish with a vengeance returning the soil to its ‘pre-flea’ N status.

The point is this. The clover/grass system is self compensating. All it requires is patience.

The requirements of an all-grass system are different to that of a clover and grass system. An all-grass system requires less P and S, because grasses have a lower nutrient requirement than clovers.

In the example below the savings in terms of the reduced inputs of these nutrients represents about $20/ha/yr. But this is more than offset by the cost of the additional fertiliser N required on an all grass system (about $200/ha).

This is a further example of the fact that it costs about 2-3 cents to produce a kg of clover-grass DM, but about 10-12 cents for a kg of all-grass pasture DM.

Consider that clover has a higher nutritional value than grasses and that milk production and animal growth rates increase with increasing clover content of the pasture.

Finally, ponder these numbers: The total input of N into the NZ pastoral farming system is estimated to be about 3.6 m tonnes of N per year. This comes from the N fixation by the clover (90%) and fertiliser N (about 10%). If this entire N input was valued on the basis that fertiliser N cost 70 cents/kg then in dollar terms it can be valued at $2.4b! If this was added to our production costs how competitive would our agricultural products be on the world market?

My personal view is that NZ must re-commit itself to the clover-based pasture system and pour as much research money as possible into developing more productive legumes which are efficient N fixers and are resistant to pest attacks.

Such research should go hand-in-hand with developing biological control agents against these pests.

· To contact Dr Edmeades or to find out more about the Fertiliser Review or his other services phone 0800 33 73 46 or visit: www.fertinfo.co.nz