Reseeded pastures have higher grass growth rates compared to old swards. This is particularly evident at the shoulders of the year when grass growth is less. How much more grass will be grown depends on the type of sward that it replaced or is stitched into. Fields with old permanent pastures with a small proportion of perennial ryegrass will grow a lot more when replaced by new perennial ryegrass.
Reseeded pastures are more responsive to fertiliser so they will give a higher return in terms of grass for every kilo of fertiliser applied, increasing efficiency. New varieties are better quality and more digestible so animal performance increases. The total cost of reseeding is around €300 to €400 per acre, while overseeding costs around €120 to €140 per acre, depending on input costs but the extra productivity would cover the cost in just two years.
Why doesn’t new grass last?
Farmers often say that after a few years the old grass returns. This is definitely evident in some cases, but it mostly boils down to the management of the soil and the sward after sowing. Perennial ryegrass needs to tiller (create new plants) to survive. In order for it to successfully tiller it must be grazed tight. If the plant is not grazed tight, particularly in spring and autumn, then the new tillers will die, resulting in a reduction in the percentage of perennial ryegrass in the sward over time. So very extensive grazing, leaving high residuals or regular cuts of silage will reduce the percentage of perennial ryegrass in the sward.
Perennial ryegrass is then replaced by weed species of grass such as agrostis, scutch, annual meadowgrass and meadow fescues – none of which have the growth characteristics or quality as in perennial ryegrass.
The second reason for new varieties not lasting is poor soil fertility. Perennial ryegrass, like any top performer, needs energy to be at optimum. You wouldn’t feed bad hay to a racehorse and expect it to win the Grand National. Nor should you expect perennial ryegrass to last in the sward when soil fertility is poor. The most important things to focus on are soil pH, phosphorus and potassium index levels and then feed the grass with nitrogen as and when it’s needed or incorporate clover and let that provide most of the nitrogen.
What’s the best method?
Numerous studies have shown very little difference between establishment methods. All methods are successful once carried out correctly. Each method has pros and cons. Ploughing can bring up stones, bury soil nutrients and release both carbon and nitrogen from the soil. Minimum tillage can take a bit more time as the old sward needs to die back fully before cultivating and seedling emergence can be a bit slower. However, there are fewer stones to pick and it’s better for the environment. Oversowing works if the existing sward isn’t that bad, but it can be hit and miss if the existing sward is very dense as it shades out the new seedlings. Its success largely comes down to management.
When is the best time to reseed?
Fields sprayed off with roundup now won’t be grazed again properly until mid-June. It’s a long time for a field to be out of production but that is the reality of reseeding. This being the glory of overseeding as there is essentially no downtime of the paddock in the grazing rotation and no spray required.
Regular, tight grazing helps the new sward to tiller. This can be hard to achieve in the autumn and early winter when land is getting wet and days are getting shorter. It also reduces the ability to get the field sprayed in optimum conditions.
What varieties to pick?
Most co-ops and merchants have their own grass seed mixtures, with and without clover. The introduction of the Pasture Profit Index (PPI) has greatly increased the amount of information on each variety. The index gives each variety an economic value based on its traits. Most mixtures will have a mix of tetraploid and diploid grass varieties but for oversowing I would recommend using tetraploid varieties as they have larger seeds.
What do most farmers forget?
The thing forgotten most when reseeding is post-emergence spray and in my view it is one of the most important parts of the process. At €30-40/acre for the spray and the cost of spraying, it is small in the overall scheme of things. The main reason farmers give for not spraying is that the grass would outcompete the weeds. However, the main problem is the dock and or thistle, and they will not be out-competed by the grass. Invariably, farmers who don’t use a post-emergence spray end up regretting it. Weeds like docks are only ever really successfully killed as seedlings. Killing a mature dock with a large tap root is extremely difficult. The best time to spray is after most of the weeds have emerged and the grass is about two weeks away from being fit to be grazed.