Fertiliser on forage crops…reduce rates when sown in August

Forage crops sown after cereals can be extremely beneficial in years where there is a fodder deficit on farms.

Completing a fodder budget is essential and if the second-cut of silage didn’t yield as well as expected or needed then sowing a forage crop may be an option.

To fill the gap in winter feeding plans farmers may decide to plant a brassica crop or a fast-growing grass and depending on the time of year the crop is planted and the amount that is needed fertiliser applications will vary.

Brassica crops are relatively easy to establish and fit well into a farm where oilseed rape is not being grown as part of the rotation.

Crops like kale sown from April onwards will have received a fertiliser application at sowing.

Crops planted from now on are less likely to reach the same yield potential and according to Teagasc’s newly updated Green Book, which was published in July, crops planted in August – after a cereal crop – can expect to have a yield reduction of approximately 30%. As a result, fertiliser application should be reduced.

Where fertiliser is being applied it is best to incorporate it into the seedbed.

This table (below) shows nitrogen (N), phosphorus (P) and potassium (K) rates (kg/ha) for forage rape according to soil index. Readers should bear in mind that applications to later sown crops should be reduced due to the lower yield expected.

Image source: Teagasc

Teagasc also states that brassica crops need sulphur (S). Approximately 15-20kg of S/ha is required and is needed most on light soils and soils with low organic matter contents. Trace elements should be applied according to soil tests where deficiencies are poor.

It is well known that brassica crops benefit from boron (B) and, similarly to S deficiency, B deficiency may be more likely on light soils.

A host of factors are at play here but, when it comes to these crops, the earlier the better for planting. Growth will be significantly slower in August than in July and in September compared with August, so once fields are cleared planting should take place.

Fertiliser application in GLAS

Farmers planting catch crops under the Green, Low-Carbon, Agri-Environment Scheme (GLAS) may graze these crops after December 1.

Crops sown as part of GLAS cannot receive high levels of fertiliser.

This table (below) outlines potential fertiliser application rates for crops in GLAS which will be grazed.

Image source: Teagasc

Forage crops are catch crops too

Remember these crops will take up nutrients left in the ground which have been left behind from the previous crop, so consider what may be left following this year’s growing season and in your fertiliser allowance before applying fertiliser.

For example, the drought during the summer may well mean that there is nitrogen in the ground that was not used by the spring barley crop.

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