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Sunday 11 December 2016

Take a leaf out of the plant physiology book

Dairy

Mary Kinston

Published 20/09/2011 | 05:00

Autumn should ideally allow pasture cover to build and offer the opportunity to extend our grazing season. However, recent weather conditions have resulted in a mixed bag of on-farm situations. This is ranging from heavy soils being water logged, to areas with good grass growth resulting in a substantial increase in pasture cover with minimal supplementation. In contrast, other farmers are unable to build up or hold the present cover as a knock-on effect of dry conditions.

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As a result, no one solution fits all and you will need to regularly walk and determine your pasture cover to review your present decisions and make any necessary change this autumn.

The aim must be to balance both the requirements of the cow (feed demand and condition score) with those of the pasture plants.

Understanding plant physiology may assist you with your decisions this autumn. Perennial ryegrass plants consist of a number of tillers connected at the base.

Each vegetative tiller is able to maintain three live leaves and as the fourth leaf emerges, the oldest leaf dies. Immediately after grazing, the growth of the roots and of new tillers stops and the tiller uses stored water-soluble carbohydrates (WSC) to grow leaf one.

Once tillers have grown between half and one full leaf, roots begin to grow and plant begins to store WSC.

As a result, pasture growth immediately after grazing is slow, and between grazing and the one-leaf stage (second leaf emergence), only accumulates approximately 15pc of total pasture yield. Growth of the second leaf to the two-leaf stage (third leaf emergence), allows new tillers to grow and growth accumulates more rapidly to 35-40pc of total pasture yield.

From the two-leaf stage to the three-leaf stage WSC reserves are replenished to pre-grazing levels, and growth will account for 45-50pc of total pasture yield. As the fourth leaf grows, sensence (death) will start to equate to growth, and tillering will reduce with an increase in stem formation as tillers receive reduced light penetration to the base of the plant.

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So, in order to optimise pasture growth and quality, the aim is to graze a sward between the two and three-leaf stage, which has implications on the appropriate rotation length.

Leaf appearance and extension rate are substantially influenced by temperature and moisture availability. In mid-summer, leaves may appear as often as every seven to 10 days, compared to 35 to 45 days during winter. Therefore, considering the three-leaf theory, it makes sense that to graze perennial at the 2.5-leaf stage in mid-summer would correspond with a commonly used 18-25 day rotation length.

It is also appropriate to lengthen rotation as we progress through the autumn to winter. A 35 to 40-day rotation length is often suitable in mid-September, though it can be useful to determine the leaf stage of your own pasture.

If, however, you are in a feed deficit and struggling to build cover, it is more appropriate to graze at the three-leaf stage (eg 40-45 days).

grazing

On the other hand, if you have built up substantial pasture cover relative to pasture demand or have heavy ground, grazing at the two-leaf stage (eg, 30-35 days) may be more appropriate to encourage grass growth and tillering.

It's important that the rotation length increases during the autumn as grazing pastures before the two-leaf stage (eg, 20-25 days) will reduce the phase of rapid growth, and reduce total pasture accumulation (increase the feed deficit), and will inhibit the tillers' ability to replenish WSC and will reduce the ability of plants to tiller and survive.

The three-leaf theory also promotes consistent residuals (3.5-5cm), especially in the last two grazing rotations. This is essential to increase tillering, promote grass growth and retain pasture quality as tillers within rejected clumps in a grazed pasture have often exceeded the three-leaf stage. Therefore cleaning out pasture well this autumn is important, although supplementary feeding and poor weather conditions can make this challenging.

Finally, if wet weather is to continue, it is important to minimise poaching. Poaching damage can affect pasture production and soils structural resilience. When grazing in wet conditions, the aim is to reduce the distance that the cows travel on a growing sward. Day-to-day management will require on/off grazing.

In the autumn, two periods of 4.5 hours' grazing after milking can achieve 95pc of the intake achieved by cows given 22 hours access.

To be effective this means not feeding cows silage while standing off, using narrow temporary lanes to access the back of paddocks, either strip grazing with a backing fence or grazing in blocks.

It is essential that paddocks have multiple entrances from well-crowned wide roadways to assist with this.

Dr Mary Kinston is a farm consultant based in Kerry. Email: mary.kinston@gmail.com

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