Targeted supplement use at the autumn break

Once the autumn break arrives, it can be difficult to know when to stop feeding supplements to your cattle.  There is a balance between trying to do the right thing for the pastures, in terms of allowing them to recover from the dry, and to reduce or stop the amount of supplementary feeding being used in the system.

In a rotational grazing system, one of the best ways to determine how much supplement to use and when you can stop supplementary feeding stock is to use the following equation. (source: Dairy NZ)

[Stocking rate (cows/ha) x pasture requirement (kgDM/cow/day) x rotation length (days)] + residual (kgDM/ha)

This equation (targeted grazing) will allow you to calculate how much feed is needed in the paddock if no supplement were provided, maintaining the rotation length at the number of days you want.  If there is not enough feed in the paddock, subtract the kgDM currently available in the paddock from the amount of feed needed, as calculated above.  This will help you to determine how much supplement is required to maintain the rotation and to minimise the risk of overgrazing by the stock.

For example: 320 head cattle on 148 ha.  You want to maintain a 30-day rotation, leaving behind 1400kgDM per hectare at the end of that rotation.  The pasture on offer has an energy value of 10MJME/kgDM.  The stock weigh 400kg liveweight and you want them to grow at 1kgLW/day.  Supplement with hay when required —  this hay has an energy value of 9MJME/kgDM

Stocking rate = 320 head on 148 hectares = 2.16 cows/ha

Pasture requirement: using the energy lookup tables in the Agriculture Victoria Drought Feeding and Management of Beef Cattle book, a 400kg steer/heifer needs 94.5 MJME/day to grow at 1 kg/day.  So, if the pasture has an energy value of 10 MJME/kgDM, the cattle need to each eat 94.5 ÷ 10 = 9.45 kgDM/ day

Rotation length: best practice is to base the length of rotation on the leaf appearance rate of the main desired species in the paddocks – for example, ryegrass has three active growing leaves at any one time.  If the leaf appearance rate is 10 days, the rotation should ideally be 30 days to allow the three leaves to grow to their maximum, which allows good root recovery and growth of the plant.

Residual:  To ensure good growth from young stock, cattle should not graze below 1400 kgDM/ha.  Grazing below 1000 kgDM/ha retards plant growth, restricts intake and increases erosion hazard. So, using the targeted grazing equation for our example, the paddock needs to have:

[Stocking rate (2.16 cows/ha) x pasture requirement (9.45 kgDM/cow/day) x rotation length (30 days)] + residual (1400 kgDM/ha) = 612.36 + 1400 = 2012 kgDM/ha for us to stop supplementary feeding.

If the paddock only has 1900 kgDM/ha when we go to put the stock in, it means we are 112 kgDM/ha short of feed that we need to make up for if we want to minimise the risk of overgrazing the plants and ensure the 1 kgLW/day growth rate of the cattle.

Let’s say our paddock was 3 ha in size.  This means we are short 112 kgDM/ha x 3 = 336 kgDM in total for the time period those stock will be in the paddock.  But we need to take into account the energy value of the supplement we are feeding as it will often be different to the paddock feed on offer.

To do this, multiply the total deficit of feed by the energy value of the pasture.  336 kgDM x 10 MJME/kgDM = 3360 MJME.

Then divide this number by the energy value of the supplement you will be using, in this case the hay as mentioned in our above example.  3360 MJME ÷ 9 MJME/kgDM = 373 kgDM of hay will be needed for the time period the stock are in the paddock (this figure doesn’t include any wastage).  With a 5x4 round of hay having roughly 300 kgDM weight, this means you would require roughly 1.2 bales of this hay, or 1.5 bales if you want to include 20 per cent wastage.

Don’t forget to regularly update your pre-grazing targets as the stock change in liveweight and the rotation length changes.