To anyone unfamiliar with dairy farming, the term ‘Magic Day’ probably won’t mean much. If you’re thinking of Harry Potter, you’ve gone wrong. To put it simply, ‘Magic Day’ marks the crucial point at which a farm’s growth rate of grass is equal to that required to feed the cows. Usually occurring in early to mid-April, (dependent on stocking rate), from this date paddocks can be shut up ready for silage (winter fodder). The problem every farmer must solve however, is just how much area should be closed for silage at any one time.
To make this decision easier, we’ve worked out an example for you to use as guidance. In regards to terminology, ‘ha’ stands for hectare and ‘dm’ for dry matter.
For this example, we will be basing our calculations on a 100 hectare farm which supports 300 cows. Each cow requires 17kg dm/ha/day. Therefore, to establish the amount of grass required daily, we simply need to multiply the number of cows by their requirement – 300 cows x 17kg dm/ha = 5100kg dm/day required daily on the farm. Therefore, ‘magic day’ is achieved when the growth rate equals 51kg dm/ha (5100kg dm/day ÷ 100ha = 51kg dm/ha).
Hypothetically, the availability of grass pre-grazing is approximately 1250kg dm/ha, which is calculated by utilising a pre-grazing height of 2750kg dm/ha down to a residual of 1500kg dm/ha. To establish the demand for each day, we just divide the 5100 kg/ha required by the 1250kg/ha available. This results in an area of 4.08ha required per day to feed the 300 cows.
Obviously, paddocks require time to grow. This growth rate however is a vital part the calculation to predict the area to allocate for silage. Measuring growth rates is achieved by using a plate metre which measures the quantity of dry matter on a paddock. For example, we have a growth rate of 70kg dm/ha (typical day on the 1st June). We divide the optimum quantity of pre-grazing grass, 1250kg dm/ha, by the growth rate of 70kg dm/ha, to give 17.85 days. With this information, we now know that the cows can graze the paddock approximately every 18 days from a pre-grazing height of 2750kg dm/ha to a residual of 1500kg dm/ha.
Finally, we need to work out the area required to continuously satisfy the daily cow demand, with a grass growth rate of 70kg dm/ha. This is done by multiplying the daily required area, which we just established was 4.08ha, by the number of days it takes to grow 1250kg dm/ha, which we said was 17.85 days or 18 days. The end result is 72.8ha required for the cows to be fed daily.
Given that this is a 100 hectare farm, all we now need to do is subtract the required 72.8ha from the available 100ha, allocating 27.2ha to be shut up for silage.
However, remember that this is only a guide, not a guarantee. To achieve as realistic a projection as possible you need to be fairly accurate in your grass growth rate predictions. It is essential that you consider factors such as heat, water, fertiliser and the previous years’ records.
Of course, the variables are different on every farm and in each scenario. At Grasslands, we endeavour to achieve a stocking rate of 3.1 cows/ha. However, research suggests aiming for a 90kg live weight per tonne of dry matter grown. Therefore with 300 cows, at an average weight of 550kg each, it would require 135.8ha to feed them, ultimately increasing the percentage of grass feed.