DairyNZ senior scientist, Kevin Macdonald.

DairyNZ’s Spring Rotation Planner (SRP) was developed in the 1980s from a set of trials by Dr Arnold Bryant and his coworkers, who were studying winter/spring pasture rotation length and milksolids production. It’s ingenious in its simplicity: you can use it to plan ahead – or to manage your way out of a feed shortage at calving time.

The SRP assigns a grazing area per day which increases each day from the start of calving until balance date. Initially it’s small because all cows are dry, but as the number of cows calved increases, so too does the area’s size. The aim is to start at the winter rotation at the start of calving, and time the fastest rotation to take place when pasture growth exceeds herd requirements.

The trials

In the first set of trials, farmlets were managed using a range of rotation lengths over the autumn and winter, so there was a large range in the amount of pasture on each farm at the start of calving (1400 to 2400kg DM/ha). At calving, all farmlets were then managed similarly. The amount of pasture on farmlets in July or August was not consistently related to total lactation production. So, why was this difference in feed on-farm at calving not expressed in a milksolids advantage?

Over the next two years different rotations were superimposed on the different winter rotations. After calving, the rotation lengths were manipulated. At one extreme, the cows were exceptionally well fed (very fast rotation); at the other, feed was rationed (very slow rotation). Bryant and L'Huillier (1986) noted that maintaining a slow rotation returns pasture covers to the target much sooner than a fast rotation during a spring deficit.

The third set of trials identified that the faster the rotation in the late winter/early spring period, the less pasture is grown. That’s because ryegrass stores its reserves in the base of the plant which, if frequently depleted, substantially reduces regrowth.

Even though feeding cows more pasture may seem like the right thing to do (fast rotation), it results in lower covers for longer and an extended length of time to balance date – so any feed shortfall should be made up (if required) with conserved feed instead.

Bryant and his co-workers identified four important systemlevel factors to optimise winter-spring grazing management: two were strategic (calving date and stocking rate) and two were tactical (autumn pasture management and the ideal cover at calving; and area allocated/day during winter and the development of the spring).

Spring Rotation Planner

• Grazing management in the first two months after calving largely determines production to Christmas.
• The Spring Rotation Planner takes the guesswork out of grazing management in the critical spring period.
• Put a laminated A2 poster on the shed/office wall to help communicate the SRP with your farm team.

Take the guesswork out of grazing management. Check out the Spring Rotation Planner today – dairynz.co.nz/srp

This article was originally published in Inside Dairy August 2018