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Do you know how to work out the pre-grazing yield?


John Donworth

John Donworth

John Donworth

What a difference a fine week makes out on dairy farms. We have gone from cows being inside day and night on heavy soils to the same cows being out day and night last week. Grazing conditions could not be better and the general mood among dairy farmers has improved no end.

What about current grass growth rates? By the end of last week, growth rates were approaching 50kg/day. This growth rate would support 3.1 cows/ha, with little or no meal in the diet. However, grass covers in the paddocks immediately ahead of the cows has not yet reached the required pre-grazing yields one would want. This situation has to be corrected before one can even talk about closing ground on the milking block for silage. Last week grass covers immediately ahead of the cows averaged out at between 900kg and 1,000kg DM/ha.

This is a big improvement on the 600-700kg that was common at the beginning of last week. By the way, do you know how to calculate dry matter per ha? Being able to calculate the pre-grazing yield for your stocking rate is part of the Dairy Efficiency Programme for this year.

So, if you have a stocking rate of three cows/ha, you must multiply this figure by cow intake. For the majority of Irish cows, the intake figure is 16kg of dry matter per day. So, if we multiply 3lu/ha x 16, we get 48kg.

Next, you need to multiply this figure by the rotation length. Let's assume a rotation length of 21 days. So 21 days x 48kg is 1,008kg. Now, we need to factor in a figure of how much grass the cows are leaving behind once they leave the paddock. This figure is called the residual. Last week, cows were leaving very little behind them as they were grazing low covers that were green to the base and, of course, grazing conditions were excellent.

So, as cows were leaving very little leaf cover behind them, in this situation the residual was 50kg. Dung pads have been cleaned out at this grazing pressure. Once we get into a more normal grazing situation out on dairy farms, the residual will be 100kg of dry matter. So, adding the 100kg to the 1,008, gives us the pre-grazing yield figure of 1,108kg. Now, since this is not an exact science, the ideal pre-grazing yield at a stocking rate of 3lu/ha is between 1,100kg and 1,200kg of gross dry matter per ha.

As you are reading this piece, grass covers ahead of cows should be becoming close to the target pre-grazing yield for your farm. We are having white frosts at night and this will slow down growth somewhat, but we are also having days of clear blue skies. The wind is north east and that will also keep down growth rates but the main requirement for a number of you is that the farm is drying on a daily basis.

Little or no silage ground is closed yet on the milking block. This should change this week as hopefully growth rates move into the 60-70kg bracket.

Paddocks closed today will be cut the first week of June. How much nitrogen should they get? I would apply 70 units of nitrogen/ac on paddocks closed today. How about phosphate and potash? Again, if they have received none this year, they will need the equivalent of three bags of 0.7.30 per acre, assuming, of course, your fertiliser plan allows you spread phosphate.

So, as the grass situation continues to improve on a daily basis on farms, milk yields have lifted significantly, by up to half a gallon in many cases. Cover per cow at 150kg is still tight and meal feeding will be required to bridge the gap until cover per cow and target pre-grazing yields are achieved.

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