Spray topping pastures at the right time, with the correct product can significantly reduce the amount of seed set from unwanted annual grasses. This is achieved by using a sub lethal rate of herbicide as the plant is coming out in head and beginning to flower. Two of the most important questions to consider when spray topping your pasture are, when to spray? & What product to use?  In order to answer those key questions, we need to understand how each herbicide works. 

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Gramoxone (Paraquat)
Paraquat is a contact herbicide that should be used to target grass species which have developed beyond the flowering stage.  Growers must be aware that any late developing tillers or heads that are still covered or shaded by the flag will continue to grow and produce viable seed.  Therefore, it is important to manage the pasture up to the correct timing for application to ensure optimum control is achieved.  Ideally delaying the application of paraquat to milky dough or dough stage can help reduce any tiller regrowth in the pasture resulting in a high level of seed set control. 

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Weedmaster DST (Glyphosate)
Glyphosate is a systemic product and needs time to move through the plant to get adequate control.  Therefore, naturally it’s a product that needs to be applied early in the growth stage of the targeted grass species.  Delaying glyphosate through to the dough or milky dough stage will reduce the level of control as many of the seed heads begin to mature before the herbicide is effective   

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Barley grass & Soft brome
Barley grass tends to mature and send up seed heads over an extended period.  Therefore, managing the pasture leading up to spray topping is an important part of its successful control.  Ensuring that the pasture is adequately grazed across the paddock will ensure the barley grass will mature and develop more evenly.  As mentioned, glyphosate should be used early on when the seed heads first emerge and before the dough stage.  This will ensure it has time to move its way through the plant and control it before it has time to mature and set viable seed.  Growers need to be aware of the risk associated with an early application of glyphosate in a legume-based pasture as the flowering time of the legume may coincide with the early glyphosate timing.  In this circumstance the application of glyphosate can significantly reduce the legume seed set and a later timing using paraquat would be a safer option.  Paraquat can be held off until after flowering up until the dough stage before control is reduced.           

 

Silver Grass
Silver grass has a more uniform head emergence then both barley grass and soft brome.  It is important to monitor the pasture closely, and if the soil is drying out and weather is warm, be wary of silver grass’ ability to progress from flower to hard seed in about a week.  Glyphosate needs to be applied in the early head stage and paraquat slightly later before haying off.  Although because of silver grass’ quick development the best bet option would be to use a paraquat application in this scenario to avoid the glyphosate failing to control seed set.     

 

Bent grass
Bent grass can be prolific in high rainfall zones and is usually a sign of soils lacking fertility.  Best control of bent grass is achieved when glyphosate is applied before the grass goes to head.  Please ensure your talk to your agronomist on the appropriate rates needed to control bent grass in pastures. 

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Follow up to spray topping

Heavy grazing after spray topping can help with a reduction of the target plant species.  Stock will generally select new growth (surviving tillers) as it has an increased palatability and increased digestibility.  The spray topped areas may also experience increased protein levels thought to be a result of the halting of grass growth and the movement of carbohydrates and proteins to the seed heads.  It is important to read product labels in regard to withholding periods concerning restocking sprayed pastures.