Thistles, nettles and docks are the main culprits on dry ground with rushes being a major problem on wetter soils, according to Teagasc and it is advising sheep farmers to act now.
There are also possible implications for land eligibility where weed infestations are very dense.
Flockowners are advised to spray when the weeds are growing actively, but before the plants start to flower. If the weeds have started to flower, or if there is a lot of older dead material left from last year, particularly where rushes are concerned, then it may be best to top the weeds first and delay spraying until there is a fresh regrowth
The newsletter also confirms that coming weeks are the most important, from a grassland management point of view.
High grass growth rates, coupled with the fact that most grass plants will want to start heading out, means that the biggest challenge on most farms will be to graze swards fully and to ensure that grass does not get ahead.
From a practical point of view this means walking the grassland area at least once a week to assess how much grass is ahead and grazing out paddocks to a height of 4cm.
Topping should be carried out, if necessary, while residency periods in paddocks should be reduced in order to maintain high levels of animal performance.
Fertiliser should be spread on a little and often basis to maintain grass quality. Swards that are not producing adequate levels of grass should be earmarked for reseeding.
The newsletter also discusses the steps which sheep farmers can take to maintain soil fertility levels.
This activity should be underpinned by a commitment to lime regularly, thereby maintaining soil pH values. If these are not achieved than flockowners will waste up to 50% of chemical nitrogen and phosphorus that is applied.