Hitting the Mark: The Crucial Role of Target Post-Graze Residuals in Pasture Health

Harnessing the benefits of accurate post-grazing management boosts pasture productivity and health.

Post-grazing residuals play a pivotal role in the sustainable management of pasture-based dairy systems. Defined as the amount of grass left in the paddock after grazing, residuals are not just a measure of what has been consumed but a crucial determinant of the pasture's health and productivity.

The management of residuals is a balancing act; it involves ensuring that the pasture is grazed sufficiently to maximise pasture production without compromising the pasture's ability to regrow and sustain future yields.

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What is the ideal post-grazing residual?

Research shows that for ryegrass, the ideal is 1500-1600 kg which is approximately 3-4 cm.

At certain points in the year, some farmers may choose to leave residuals a bit higher or lower but it is important to know that

1. The closer to the ground that cattle have to graze, the slower their intake will be and vice versa.
2. The most important factor in residual management is consistency.

Evidence suggests that most farmers are suffering hidden losses by not properly managing residuals. In fact, one study showed that 48% of grazing events missed the recommended post-grazing targets

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Consistency is Key

Above all else, hitting the same post-grazing residuals each round is the easiest way to preserve pasture quality and persistence. One of our leading farmers summed it up perfectly, stating “whether residuals are too high or too low is irrelevant if you’re not aiming for consistency”.

When a paddock is grazed lower than the previous round, the animals will be consuming lower quality feed, which negatively affects production. When a paddock is grazed higher than the previous round, the cattle will be leaving stem material resulting in a decrease in pasture quality when animals come back to graze this pasture in the subsequent rotation.

Therefore, the most important first step is to learn to ensure accurate measurement and timely adjustments.

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Grazing too low = harm pasture persistence

Overgrazing commonly occurs when feed has been under-allocated, causing the cows to graze the pastures lower than the target residual.

The other instance when grazing too low occurs is when the pre-grazing cover is lower than it should be, causing the cows to bite lower down, eating more of the base of the tiller.

Grazing pasture below the ideal residuals affects the pasture persistence by damaging persistence and slows regrowth. This issue stems from the leaf stage of the plant; grazing too low reduces the green area, effectively diminishing the plant's 'solar panels' needed for regrowth. Consequently, the plant is forced to rely on its reserves for energy, leading to a depletion of these vital reserves.

In one case study, pastures grazed to a residual of 1300kgDM/ha or lower experienced a 20% decrease in growth compared to those with 1500kgFM/ha or higher. This reduction in yield amounted to approximately 2.4tDM/ha, equating to a loss of around $600 in profit per hectare. (2)

Not only does this have a negative effect on pasture health, but a 2013 study demonstrated that grazing residuals down below targets during early lactation had a negative effect on milk yield, protein concentration, and animal body reserves. This is likely caused by the expenditure of the extra energy required to graze lower residuals.

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Grazing too high = decrease quality and miss out on quantity

It is a common misconception that leaving high post-grazing residuals is better for pasture quality. Based on research, the opposite is true. Avoiding high post-grazing residuals is crucial for maintaining pasture quality.

One study showed that when perennial ryegrass residuals exceed 1500 kg dry matter per hectare (DM/ha), the herbage's quality diminishes, particularly in terms of crude protein and organic matter digestibility. High residuals often lead to an increase in reproductive tillers, which are less digestible than vegetative tillers. Additionally, excessive residual matter can impede the regrowth of more nutritious, new green leaves, negatively impacting the pasture's overall productivity and forage quality.

This effect is even more pronounced when paddocks have noxious weeds or less productive grass species. Cows will first eat the highest quality pasture and leave more low-quality / weeds to proliferate next round. Over time this can cause non-desirable pasture species to outcompete higher quality ones.

In fact, to counteract the loss of quality from high residuals, ‘topping’, which involves mowing a pasture after a grazing to even out the residuals, is a recommended practice to remove biomass and regain control and pasture quality.

If that is not motivation enough, when you leave high residuals, you also are missing out on pasture.

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Consistently leaving residuals just 100 kg over target would mean missing out on $380 worth of pasture / ha / year.

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Measuring Residuals

The only way to understand whether you are hitting your residuals, is to measure them. This is commonly done with a plate metre, walking a diagonal across the paddock and taking measurements at every step for 30 measurements. Plate metering does not give reliable results on pugged paddocks because of the uneven ground.

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Easy with Halter

Halter’s Photo APC tool makes it easy to measure post-grazing residuals. Just snap a few photos of the ground as the cows leave paddock and the break fence area will automatically update with the accurate post-grazing cover. These photos, which can be taken by anyone on your farm, are interpreted by another of Halter’s artificial intelligence models, which has been trained on over 10,000 images of residuals on a range of dairy farms.

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References

1. Mccarthy, S. et al. 2014. “Opportunities to Improve Grazing Management.” Proceedings of the New Zealand Grassland Association 76: 75–80.
2. Chapman et al. (2015). Hidden dollars in grazing management: getting the most profit from your pastures.
3. Clarke, Brian, Danny Donaghy, and Marie Casey. 2021. “Working with Dairy Farmers to Improve Their Pasture Management Skills through Better Understanding the Principles of Ryegrass Growth.” NZGA: Research and Practice Series 17. https://www.nzgajournal.org.nz/index.php/rps/article/view/3462 (November 23, 2023).