Agronomy Advice
September 01, 2023

Grassland Fertiliser Advice for September

By: Philip Cosgrave

With above average grass growth rates across the country, the focus should be on making the most of this grass as we enter September. The purpose of autumn grazing management is to increase the number of days at grass and animal performance, but also to set the farm up during the final rotation to grow grass over winter and provide grass the following spring.

Farmer in a field with cows in the back grazing
Farmer in a field with cows in the back grazing

Because grass remains leafy, rotation length can be extended during September. The focus of this period is to build pre-grazing covers, targeting covers of 3,500 kg to 3,700 kg DM/ha in mid-September. Be careful not to allow covers build beyond 4,000 kg DM/ha for grazing, as utilisation is poorer. We want to avoid taking paddocks out for silage after the start of September, as these paddocks won’t have enough time to re-grow to make a significant contribution to the last rotation.

If Autumn nitrogen is going to be spread, it should be spread in September. As you can see from the graph, the growth response to each kg of N for October applications is likely to be significantly lower, especially into the latter half of October and may not be economical. It’s best to grow a bank of grass now and then manage grazing allocations to optimise utilisation.

Our Calcium Ammonium Nitrate based fertiliser YaraBela Nutri Booster with sulphur and selenium at 120kg/ha is an appropriate application rate in September for a farm with high stocking rates. If P & K needs topping up, then a quality NPKS such as YaraMila EXTRA GRASS at 110 kg/ha should be used.

Drier or ‘earlier’ paddocks should be grazed from mid-September and then closed off from October onwards. Regrowth on these parts can be carried over the winter months for grazing first in the spring.

Autumn applied Fertiliser N Bar Graph

Teagasc 2019


P & K top-up

Now is a good time to review fertiliser and slurry applications this season. With a particular emphasis on potash (K). Where K inputs didn’t cover K offtakes, aim to top up to maintain soil K fertility. For those trying to build their soil K index on fields or paddocks, autumn is the time to apply build-up rates. For intensive grazing on soils with a K index of 2, we recommend 20 kg/ha of K2O, while at index 1 we recommend 30-50 kg/ha. Take account of any bales removed from those paddocks during the season. You can calculate the K2O offtake per ha from bales by multiplying the number of bales taken from a paddock by 8 kg (the amount of K2O in a bale weighing 900 kg at 25% DM), and then dividing this figure by the size of the paddock in ha’s. This is where it can be useful to use a silage grade fertiliser like YaraMila Silage Booster with its higher potash content in this scenario.

Example: How much extra K would a 2 ha grazing paddock need above maintenance if 9 round bales of silage was removed.

Answer: 9 bales x 8kg divided by 2 = 36 kg/ha K2O