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Dr Danny Donaghy, group research leader for dairy at the Tasmanian Institute of Agricultural Research, recommends the two-to-three-leaf principles for pasture management, as he believes that it is the easiest and most accurate technique.

He says that with day rotation systems, there is no flexibility or learning and that farmers get it wrong about half the time. The height measurement has the same weaknesses and is affected by many factors. Consequently, grazing management is again miscalculated about half the time.

Measuring kilogram dry material per hectare is even harder to manage and affected by many factors. Farmers also get this measurement wrong about 30% of the time. He thinks farmers can get grazing management right about 90% of the time by grazing between the two and three leaves/tiller or earlier if canopy closure occurs.

Leaf growth reflects the ryegrass plant’s energy status and also affects regrowth, survival and quality. A spreadsheet cannot accurately determine whether a pasture is ready for grazing – you need to be in the pasture and looking closely at the plants. Using leaf-stage and canopy-closure observation to determine when a pasture is ready to graze works in both tropic and temperate climates. Canopy closure works better than leaf stage and occurs when the ground or pasture base directly below the canopy cannot be seen. If canopy closure occurs in more than 25% of the pasture, there would be mainly high stem-growth rates, which leads to deceptive measures from platemeters. Future growth is depressed with high stem-growth rates as tillering declines; pasture quality also declines as the fibre increases; and the cows will eat less unless they are starving.

Plants partition energy to maintenance first, then to the growth of roots, leaves and tillers and finally to energy storage. The metabolisable energy (ME) quality does not change significantly between the one and three leaf stages. Overgrazing of pasture does not leave much energy for the roots and tillering and leads to further pasture degradation. The plant’s energy is stored in the 4 cm of the tiller and the base. Consequently, grazing and/or cutting reduces the amount of energy available to the plant. Overgrazing causes the plant to put all its energy needs on hold until it reaches the three-quarter-leaf stage. Short rotations without mineral supplementation on young pasture generally create plants with too much potassium and excessive protein. Although neutral detergent fibre (NDF) in a plant changes with stress levels and height, there is no clear relationship between NDF and leaf stage until the three leaf stage.

The ME decreases after two leaves, and even under good management, it drops more quickly after two leaves when combined with canopy closure. The ME drop is also steeper when the pasture is stressed and therefore a longer rather than shorter rotation is required for recovery.

Faster rotations at less than two leaves leads to lower energy levels, decreased pasture production and imbalanced mineral levels (high N and K, low Ca and Mg). It also leads to decreased tillers, leading to a further drop in production and decreased persistence. The root system is decreased, weakened and becomes shallower, which decreases water and N efficiency and can cause N stress. This is because the water and N that is applied quickly moves below where the plant can reach it.

Finding the right balance
The pasture can be killed by grazing hard in a couple of quick, hard rotations. There is a narrow window of grazing opportunity. With slow grazing rotations and pastures with more than three leaves, there is high production and a good mineral balance but protein levels decrease, high fibre reduces the overall feed value and shading leads to fewer tillers and eventually fewer roots. Short residuals reduce the storage area for energy causing regrowth to slow. There is less total production, reduced persistence and the cows are forced to graze with greater intensity, which reduces intake.

By finding the right balance you can optimise your pasture usage and provide your cows with the best pasture.

Phone +27 (0)33 330 6698 or e-mail iqnutrition@­iafrica.com for moreinformation.