Beneath the surface with David Law Forward Farming

New Zealand farms have reputations as lush, green properties that grow enough grass to enable cattle to be fed on pasture alone.

However, increasing temperatures have reminded farmers they should never take pasture for granted. Indeed, in this day and age, an unprepared farmer will struggle to fully feed his herd on pasture, instead relying heavily on palm kernel and Urea to either supplement the pasture, or boost its growth.

A far better option for both the farm and the farmer’s pocket is to learn better pasture management skills – so grass gets the opportunity to grow to its full potential and animal health problems can be averted.

A key component of pasture management is getting rotation length right. Rotation length is the time it takes for cattle to go around the farm, grazing a planned area per day, and return to the first paddock grazed.

Here are some points to consider when establishing a suitable rotation length.

Rotation length should be adjusted according to the speed at which grass is growing. For example, at times of the year when growth rate is quick, like spring, rotation length should be quicker so the cows can keep up with the grass growing ahead of them.

If cows are leaving more than the planned excess after grazing, called residual, then areas need to be dropped out for silage.

If the established rotation length is showing the cows are leaving less than desired residual behind, then the diet needs supplementing with extra feed in the form of silage, hay, crops or meal and the rotation needs lengthening.

While growth rate is slow it is pointless to give cows more area, which in turn speeds up the rotation and gives even less time for grass to return to the desired pre-graze level.

In times of slow growth more supplements can be fed on a daily basis to extend the rotation length to match up with the growth rate.

To establish an ideal rotation length, you need to do the following.

First, establish an ideal pre-graze level – for example, 2800 kgDM/ha. Then establish an ideal residual level – for example, 1500 kgDM/ha.

Subtract the residual from the pre-graze to give the available drymatter – for example, 2800kgDM/ha minus 1500kgdm/ha equals 1300avkgDM/ha.

The available drymatter divided by the daily growth rate – for example, 65kgDM/ha per day – gives the ideal rotation length for the growth rate: in this case, a 20-day rotation.

Establish herd demand by taking feed required by cows and multiply by number of cows– for example, 18kgDM/cow x 450 cows = 8100kgDM.

Then take the available dry matter of 1300 average kgDM/ha and multiply by the total area available to graze per day – for example, 6ha – to establish allotted feed for the herd per day. The equation: 1300kgDM x 6ha = 7800kgDM.

The daily herd demand of 8100kgDM/day, minus the available 7800 kgDM/day, leaves a deficit of 300kgDM.

My conclusion here is that to maintain a 20-day rotation at target levels, 6ha of grass and 1.5 bales of silage are required – with one bale being 200kgDM.

As pasture growth rate reduces, the rotation length will need to be increased; even in dry periods, the grass still grows.

The role of residuals remains incredibly important: if you reduce residuals and graze your farm to the ground, it is important to note the residual will act as a blanket to protect soil drying out.

As we move from drought conditions to rain and new grass growth in autumn, pasture management practices are subject to many variables – and a little bit of measuring and planning does make life easier. Please contact me if you require further information.