"It doesn't really compromise pasture quality because the cows tend to graze out clover swards cleaner than grass ones due to the higher palatability of clover," she said.
The researchers also suggest that farmers could offer a high fibre feed such as hay in the field or increase the rate of anti-bloat oil in the water if the cows are going into a paddock with a particularly high clover content, although neither of these measures are being used in the trials.
The researcher estimates that the annual cost of providing the anti-bloating oil at €0.24/cow/day.
One of the most popular varieties of clover used by farmers in the past was the large leaved cultivar called Aran. It had impressive performance, partly on the back of its aggressive growth.
However, this aggressor growth was a double-edged sword for farmers, who found that Aran often out-competed the ryegrass in the sward.
The later spring growth and lack of persistency left farmers with open pastures that were under-performing compared to their grass pastures in the spring.
The varieties being used in these trials are medium-sized leaf cultivars called Chieftain and Crusader. They do not out-compete the grass, although they have increased their proportion of the sward from 25pc last year to over 40pc this year. However, the researchers point out that this could be an effect that is particular to this year.
In addition, these varieties are believed to be quite persistant.
However, the biggest factor in maintaining clover in the sward according to the researchers is implementing the right kind of grazing regime.
"A decade ago cows would have gone into higher covers. The residuals were also higher, which meant that clover got shaded out of the sward.
It does much better in modern grazing systems with rotation lengths of 20-21 days, where grass is eaten down to 4-4.5cm and maximum covers of 1,600kg/ha," said Dr Hennessy.
"Older research suggestes a clover crash after five years. The jury is still out on whether this will happen in these trials but so far, so good, even when we are using full rates of fertiliser at 250kg/ha.
"Even if we do need to come back in with some over-sowing of clover after year five, I don't see it being a big drawback, relatively speaking," she said.
"It would cost less than €100/ha to re-establish clover in the sward, which could be less than 20pc of the extra returns that it generates every year.
"We've already seen this method work successfully on the Solohead research farm in west Tippeary," she said.
Low spring growth
The last major drawback with clover is its lower productivity in the spring compared to productive ryegrass pastures, a fact readily acknowledged by Dr Hennessy.
However, she believes that there are cost effective ways to combat this downside.
"There's no doubt that clover is slower to get going in the spring, but it compensates by producing more than the grass at the peak of the growing season," she said.
"Basically the farmer needs to cut more silage in the middle of summer and then use this to feed out during a feed deficit in the spring.
"Yes, the cost of silage is twice that of grazed grass, and probably even more so in the spring when fresh grass is at its most valuable to the farmer, but it is still outweighed by the benefits of the extra drymatter yield and extra milk solids per hectare."