Lesley Irvine, TIA
A hardy group of farmers attended the Dairy On PAR spring management field days in September. While the weather was not the best, there was plenty to discuss with guest speakers Frank Mickan (Agriculture Victoria), and James Hills and David McLaren (TIA).
At the field days, David and James outlined the checks that should be undertaken with irrigation systems NOW to make sure everything is ready to start irrigating at the right time.
It ends up very costly if irrigation systems are not started at the right time. Modelling has shown that for each day that start-up is delayed, there is a loss of 105 kg DM/ha over the season. So, if start-up is delayed by five days, the amount of pasture foregone is 0.5 tonne DM/ha. For an irrigated area of 50 ha, this is 25 tonnes of DM lost.
Irrigation systems checks
For centre pivots, you should:
There is a really useful document on the DairyNZ website called “DIY Irrigation Evaluation” which takes you step-by-step through some of these irrigation checks. Undertaking these checks ensures your system is ready to go, operating efficiently and applying the correct amount of water.
Starting at the right time
Apart from checking your system is ready to start when needed, you need to be able to work out the right time to start watering. A common mistake is to overestimate the amount of water the soil can hold. If you know your soil texture you can work out its water holding capacity and you are then able to calculate when to start irrigating. Table 1 shows some common soil types and the amount of readily available water held.
Table 1: Amount of readily available water in soils of various textures
Readily available water (mm/cm)
Readily available water in 30 cm soil
Adapted from Soil health for farming in Tasmania by Bill Cotching.
The majority of pasture roots occur in the top 30 cm of soil, so to calculate the amount of soil moisture available for pasture growth, it is necessary to multiply the amount of readily available water the soil can hold by 30 cm. For example, a sandy soil can hold 0.3 mm/cm of readily available water. Multiply 0.3 mm/cm by 30 cm and we determine a sandy soil can hold 9 mm of readily available water in the root zone.
Moisture is lost from the soil and crop through evapotranspiration. If the average evapotranspiration rate is 3 mm/day, it only takes 3 days (9mm ÷ 3mm) with a sandy soil for all the readily available water to be used and the plants to suffer stress and reduced growth rates. On a clay loam soil with an evapotranspiration rate of 3 mm/day, it will take 8 days (24 mm ÷ 3 mm) to use all the readily available water. It is important to understand that the numbers in the right hand column of the table represent the maximum amount of water that can be held by the soil. Even if there is 50 mm of rain, a sandy soil will still only be able to hold 9 mm of readily available water in the top 30 cm. All the ‘excess’ water will either run off the surface or drain below the top 30 cm and be inaccessible to the pasture.
How do you use this information to work out irrigation start-up time? Basically, you just need to go back to the last significant rainfall event (where it rained more than the water holding capacity) so that you know the soil was holding the maximum amount of readily available water that it could. You then just subtract the amount of water lost through evapotranspiration each day and add in any rainfall events greater than 2 mm. Table 2 shows an example of a water budget.
Table 2: A water budget
Readily available water
Add 0.2 (ignore first 2 mm*) and subtract 1.2
Add 6.2, subtract 1.4
Add 1.2 (ignore first 2 mm) and subtract 2.3
No data so subtract av. 1.9
Add 10.8 (ignore first 2 mm) and subtract 1.6
Add 7.6 and subtract 1.6
Add 7.2 and subtract 1.8
Add 3.8 and subtract 2.2
Add 5 (ignore first 2 mm) and subtract 1.6
Add 2.8 and subtract 2.3
Add 1 (ignore first 2 mm) and subtract 1.2
Add 8.8 (ignore first 2 mm) and subtract 1.9
Add 2.8 and subtract 2.8
Add 6.4 (ignore first 2 mm) and subtract 2.8
*The first 2mm of a rainfall event is not counted as it is considered ineffective.
Rainfall and evapotranspiration for the water budget was sourced from www.bom.gov.au using data from the Wynyard weather station. The soil texture used was clay loam with a maximum readily available water of 24 mm in the top 30 cm.
The irrigator must be started in time for the whole area to be irrigated before the readily available water reaches zero. If you don’t start until the water budget reaches zero, most of the irrigated area will be experiencing stress and reduced growth rates by the time you water.
The alternative to using a water budget is to install a soil moisture measuring device. There are a whole range of these available – your irrigation supplier should be able to provide advice on the options available.
Making quality silage
In Tasmania, a large proportion of annual pasture growth occurs in spring. In order to maintain a high pasture quality for grazing cows, it is generally necessary (unless other strategies are in place) to drop paddocks out of the grazing rotation for silage and/or hay production.
If measuring pasture growth, the below example can be used as a guide to decide on the area that can be taken out of the grazing round.
Cow numbers: 460
Milking area: 200 ha
Stocking rate: 2.3 cows/ha (460 ÷ 200)
Cow requirements per hectare: 37 kg DM/ha (16 kg DM/cow x 2.3 cows/ha)
Pasture growth (measured): 70 kg DM/ha
Area needed for grazing: 53% ((37 ÷ 70) x 100)
Area that can be taken out for silage: 47% (100% - 53%) or 94 ha (200 ha x 47%)
The area calculated for silage production can be progressively taken out of the rotation to maintain grazing quality. If pasture growth rates increase – or decrease – the area to be cut for silage should be adjusted accordingly.
Pre-grazing covers and post-grazing residuals should be monitored closely and as soon as one or both of these starts to get above target, paddocks should be dropped from the grazing rotation.
If paddocks that are dropped out of the rotation are cut early (leafy and before seed head emergence), silage quality will be very high and paddocks cut will slot back into the next rotation.
Cutting pasture early gives the best potential for a high quality silage but the rest of the silage-making process also needs to be managed well. Particularly, getting a fast wilt – ideally 24 hours – but certainly within 48 hours, is important. Obviously, timing mowing with good weather predictions is important in achieving this. Using a mower conditioner to mow or a tedder as soon as possible after mowing will make the pasture wilt faster. Make sure there is good communication with contractors regarding when and how much is being mowed.
Finally, silage needs to be wrapped/covered as soon as possible and it is critical that plastic is checked for holes and repaired quickly – losses due to mould growth can be huge.
There is a lot more information about making quality silage in the TopFodder “Successful Silage” manual which is available online at no cost.