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Using Calculations To Manage Lamb Grazing

6 August 2018

Photo from Farmers Weekly

Based on conservative projections, we can calculate the target stocking rate for growing lambs over the next month to maximise performance off grass.

To start with, we need to estimate intake at grass; we estimate this based on a percentage of bodyweight. For 25kg weaned lambs targeting 200g/day growth, we need to provide around 5.8% of bodyweight, which equates to 1.45kg DM/day.

Measuring grass in kilograms of dry matter per hectare using a sward stick helps calculate the feed available. For example, if the field is around 5cm in height – equating to 1,880 kg DM/ha – this helps make grass more comparable with other feeds – the dry matter content in grass is substantially lower than other feeds (12-30%).

We need to deduct how low the grass should be grazed (the residual). For grazing lambs this should not be lower than 4 cm (1,650kg DM/ha). Therefore, the feed available per hectare can now be calculated (1880 kg DM/ha – 1650 kg DM/ha = 230 kg DM/ha).

How quickly grass is growing is highly dependent on the site and its management. Grass growth, based on industry information at the moment (see AHDB Forage for Knowledge), for a Stirlingshire Dairy farm is 35kg DM/ha/day at week ending July 20th 2018. For the purposes of illustration let us assume a dry weather growth rate on an average site of 17.5 kg DM/ha/day or 525kg DM/ha in total for a month. This is added to the available feed above (230 kg DM/ha) to give total feed (755 kg DM/ha).

Divide this by 30 days to give the feed available per day (25kg DM/ha/day), multiply this by estimated utilisation (60% is a good starting point). Then divide by the animal requirement (example, 1.45kg DM/lamb/day) to give a potential stocking rate over the period of 10 lambs per hectare or 4 lambs per acre.

Example from above:

(25kg DM/ha/d x 60% utilisation) ÷ 1.45kg DM/lamb/d = 10 lambs/ha

Doing the calculations now, helps understand how many stock the land can support.

Poppy Frater

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