stock demand, average cover and rotation length – while maintaining cow production. Starting off with stock demand, we need to know the area that is available to graze as well as stock requirements.” Taking an example herd of 120 cows with a total area of 30 ha available to graze gives a stocking rate of 4 cows/ha. If the cows are out day and night, a target intake of 16 kg grass dry matter (DM) is assumed with the remainder of the diet fed as concentrate in the parlour.
Grass supply and demand Grass demand is a function of stocking rate multiplied by grass intake, so in this example 64 kg DM/ha/day (4 cows/ha x 16 kg DM/cow). Therefore grass needs to grow at 64 kg DM/ ha each day to meet the herd’s requirements. Then taking a closer look at grass supply, which needs to be measured as growth rate on a weekly basis. The most com- mon tool for measuring grass is the rising plate meter, which measures compressed grass height and converts it to kg DM/ ha through a calibrated equation. Even though growth rates reflect what has happened in the past, it forms the basis of decision-making for the future.
Actions to take will depend on what likely grass growth rates will be in the next week. The average farm cover is an indicator of the quantity of grass on the grazing area. In the main season this should be maintained at around 2,200 to 2,300 kg DM/ha. Post-grazing residuals in well-managed swards should be around 1,600 kg DM/ha. Target pre-grazing cover is calculat- ed by allocating the daily herd demand (120 cows x 16 kg DM = 1,920 kg DM) to the daily grazing area. On a 20-day rotation this would be 1.5 ha (30 ha/20 days) in this example, giving a herd demand of 1,280 kg DM/ha (1,920 kg DM/1.5 ha). Therefore, pre-grazing cover should be the post-grazing cover plus the herd demand which is 2,880 kg DM/ha in this example. The aim should then be to maintain an average cover during the mid-summer period of 2,240 kg in this example (2,880 +1,600 = 4,480/2 = 2,240 kg DM/ha). Rotation length is the number of days it takes to complete one cycle of the grazing area. During the main grazing season this should be 20–24 days. The reason for targeting a 21-day rotation is due to the ryegrass plant maintaining three live leaves at one time. Under ideal conditions during the main
▶ DAIRY GLOBAL | Volume 8, No. 3, 2021
Well-managed, high-quality grass is benefi- cial to the ani- mal, the sward, the environment and the farmer.
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