Former AGCSATech Agronomist Andrew Peart provided his thoughts on some of the key considerations to ensure you get the best results from your spray applications.
Despite great advances in pesticide formulations, gaining acceptable control of pests, or in this instance weeds, seems to be getting more difficult in recent times. The term resistance is seemingly becoming more and more common and with certain species such as Poa annua this may well be the case.
However, "as turf managers are we doing as much as possible to get the best result from our spraying applications?"
There are many aspects that must be considered to gain successful weed control. Among them are correct identification, selecting the appropriate herbicide, the timing of application and ensuring the equipment used to deliver the active ingredient is appropriate and operating at its optimum.
Obviously, it all starts with the correct diagnosis of the weed. In turf situations this can be difficult, especially if it is a grass weed which may not produce a visible seed head due to mowing frequency. If this is the case, a plug should be taken of the suspect plant, potted up and then allowed to grow and produce a seed head to make identification easier. Grasses can also be identified through the use of a taxonomic key which uses specific plant characteristics and parts (such as vernation, auricles, collars and ligules) to identify them. Broadleaf weeds generally are identifiable by their foliage characteristics and it is rarer where a flower is needed to correctly them.
Once correctly identified, selecting the appropriate turf-registered herbicide is crucial and where possible taking into account the chemical group to try and limit the possibility of resistance. With herbicides, the chemical groups (modes of actions) are represented by a capital letter. Any herbicide with the same letter has the same mode of action and their continual use should be avoided.
For example, the herbicides containing the active ingredients trifloxysulfuron (Monument®), rimsulfuron (Coliseum®), foramsulfuron (Tribute®) and iodosulfuron (Destiny®) are all Group B herbicides and rotating between these chemicals will not avoid resistance of the weed trying to be controlled. A similar scenario is also possible with commonly used broadleaf herbicides that are contained within Group I.
The other crucial component of the chemical selection is the quality of water that it is to be mixed with. The pH level of the water is most critical and ideally should be between 5.5 and 7.5. The use of buffering agents should be incorporated into the water if it is outside this range. The use of dirty water, or that with a high percentage of suspended solids, should also be avoided particularly when using glyphosate as this can bind to the colloidal particles and be far less effective on killing the weeds.
The timing of chemical applications is also crucial. Many chemical labels under the heading ‘Direction of Use’ state that the product should not be used on turf or weeds that are under stress. Others state not to be used on waterlogged soils. Some even may state that applications should not be made if heavy rainfall is forecast in the following 24 hours. Taking note of these directions is crucial in obtaining spraying efficacy.
Of most importance though is not spraying when plants are under stress. When a plant is under stress its stomata will be closed and not receptive to herbicide applications which are foliar absorbed. An example I once saw was a sportsground that had been oversown with ryegrass which was to be sprayed out before line-planting in summer. As the ryegrass was not the desired species it had not been irrigated but when sprayed with glyphosate there was no kill. After the irrigation was run to promote the new couchgrass sprigs, the ryegrass bounced back to the point where it then out-competed the couchgrass.
Ensuring the chemical reaches the target effectively is the next main consideration. Whether it’s a hand sprayer, small pedestrian sprayer or a large boom sprayer, correct calibration of equipment is essential. As most chemical labels state, the amount of chemical that is required to be applied per hectare and the correct water volume that that chemical must be mixed with is imperative. This can only be achieved through calibration.
Some spraying equipment will provide you with a water volume required based on the speed of travel, the spraying pressure and type of nozzle being used. However, what it won’t tell you is how even your application is (i.e.: the exact amount of mixture coming out of each nozzle). Poor spraying results often occur due to poor spray application.
Blocked nozzles are an obvious issue, however, what may be less obvious is poor nozzle performance of partially blocked or under-performing nozzles that may be delivering less active ingredient than actually required to kill the target plant. The only way to completely ascertain the effectiveness of nozzles is to check the output from each nozzle. This will then also provide you the total amount of water and chemical that is required to spray a hectare of turf.
The best way to check nozzle performance is to place a bucket under each nozzle and run the sprayer at the desired pressure for a set period of time, say one minute. The volume collected from each bucket should then be measured and recorded. The total and an average output from each nozzle can then be calculated. Any nozzle that varies by more than five per cent from the average should be replaced and then the test re-run. The issue may be that if uniform wear has occurred in most spray nozzles, the newly replaced nozzle may also be outside the five per cent tolerance and as such all nozzles should be replaced.
Once the total spray volume is known for a set period of time at a certain pressure, calculating the amount of water applied over a hectare is relatively straight forward. The effective width of the boom is calculated by the number of nozzles present multiplied by the distance they are apart (usually 0.5m). Travelling speed is either known from a speedometer or on older machines knowing the time it takes to travel 100m in a certain gear and revs and calculating the speed in kph from that (3.6 x 100 / time it took in seconds to travel 100m). The application rate (L/ha) then becomes 600 x spray output (result obtained in L/min from all nozzles) divided by speed (kph) x boom width (m).
Knowing your actual application rate and knowing all your nozzles are within five per cent output still does not guarantee even distribution. Boom height above the ground is paramount as having the boom too close to the ground will deliver too much spray underneath each nozzle, while having it too high may mean the spray will not completely reach the target. The best way to assess the correct boom height is to run the sprayer over a hot surface, ideally concrete, and watch the uniformity at which the water evaporates.
Nozzle selection can also be critical to obtaining maximum spray performance. It is very rare that ideal spraying conditions occur which minimises the impact of drift. The use of flat fan nozzles no doubt provides the greatest leaf coverage for broadleaf type applications, however, due to the amount of very fine droplets produced they are susceptible to excessive drift.
The use of air induction nozzles has dramatically reduced the impact of drift through the incorporation of much larger droplets in the spray spectrum. At 3 bars of pressure, a normal 110 degree flat fan nozzle produces 30 per cent of its spray volume with droplets smaller than 150 microns, whereas an air induction 110 degree nozzle has only 5-7 per cent smaller than 150 microns.
The type of nozzle can also dramatically impact on coverage and as previously mentioned the regular flat fan is excellent on broadleaf type plants. Unfortunately not many grasses can be considered broad, especially when viewed from above and also the density of turfgrass plants (especially on greens) is far greater than pasture grasses.
In recent times the introduction of the XC nozzle by Syngenta has better addressed the issue of reduced drift and also uniformity especially on sloping type ground where excellent distribution is still achieved when the boom height is reduced to only 30cm above the turf sward. Lastly, the rearward facing alignment of the nozzle has improved leaf coverage. The TeeJet air induction turbo twinjet is also another nozzle that has tried to address the issue of both drift reduction and leaf coverage with a bi-directional spray output to maximise leaf coverage.
The ability to maintain pest-free playing surfaces is paramount to the expectations of users of natural turf playing surfaces and these days that is generally only achievable with the use of pesticides. Ensuring these are applied correctly will ensure they have the best opportunity to achieve their desired result while minimising any potential pest resistance issues, harm to the operator or the environment.
Spraying equipment and water quality must be continually checked to ensure optimum performance, and the use of new technology embraced to ensure the best possible outcome. For those on golf courses, the use of spray strips on a practice fairway or even practice green could provide some valuable information on not only chemical efficacy but possibly even nozzle selection.