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Managing Autumn Grass

Managing Autumn Grass

In fertilising the grazing sward until close out it is important to not apply the 27 to 35 units per acre as straight nitrogen. This will result in protein readings of 22% to 27% which a significant excess of the requirements for 16 to 23 litres of milk per cow per day.

The requirement in the total diet is approximately 15% to 17% crude protein. This excess protein has 3 issues:

  1. It will mobilise body condition when one is trying to increase it to have a BCS = 3.0 at drying off.
  2. The cow uses energy to get rid of excess nitrogen equating in 1 to 1.5 kg of concentrates.
  3. A drop in milk lactose. A drop of05% in milk lactose is large and this can happen in a collection. This indicates sub-clinical ketosis. The milk lactose in the last 100 days of the lactation should be in the range of 4.65% to 4.73%, the latter at this stage.

To address this, it is important to use a fertiliser with potash (24:2.5:10) at a rate of a bag to the acre. It bulks up the sward greater than straight nitrogen and gives a higher dry matter sward with a higher nutrient density in particularly. The sugar is particularly affected as per the analysis below with using straight nitrogen.

The concentrate supplementing this grass should contain 40% cereal. A low sugar reading, which is critical for the cow to utilise the nitrogen and maintain the lactose %, milk yield and milk protein, is potentially missing in the sward using straight nitrogen. The concentrate should supply 32 grams of magnesium to prevent grass tetany and act as an effective buffer.

Forage Analysis:

1 2 3
Date 15/07/2016 28/07/2016 12/10/2015
Fertiliser Type CAN CAN 19:O:15
Quantity – Bag 1 1.25 1
DM% 25.6 17.6 17.5
CP% 17.4 21.2 19.7
DMD% 74.2 73.9 80.0
ME MJ/KgDM 10.7 10.7 11.7
NDF % 41.2 48.5 31.6
Sugar % 13.0 5.2 20.5
Nitrate Nitrogen% 0.06 0.13 0.15


With sample 1 there were drought conditions and partial TMR feeding in practice. The farmer wanted more grass so decided to apply extra nitrogen, when the issue was moisture deficit. Fertiliser was applied, it rained resulting in sample 2. The buffer was removed but milk yield dropped by 1 litre per cow and milk lactose dropped by 0.07%. This was a result of the drop in sugar of 7.8% equating to 2 kg of barley taking 16 Kg DMI of grass. Sample 3 shows the nutrient density achieved with fertilising with potash in the autumn.

Written: Pat O’Byrne 17th August 2018