How the Pasture Profit Index works works
Published 08/04/2015 | 02:30
Selecting grass varieties is a process that's undergone somewhat of a revolution over the last year, with the unveiling of Teagasc's long awaited Pasture Profit Index (PPI) ranking system for all grass varieties.
The index works on the same principle that is used in the EBI and €uroStar beef indexes.
Following extensive research over the last decade by a team led by Moorepark's Michael O'Donovan, a formula has evolved that places different values on the key characteristics of each grass variety.
"This formula will allow us to have the best recommended list in Europe," said Dr O'Donovan.
The first element in the formula is the amount of grass that any particular variety can grow. But Dr O'Donovan was not content to just take an overall yield for each grass.
Extra weightings are given to varieties that are able to produce more grass on the shoulders of the grazing season, because these are the periods where there is greatest scope to replace expensive concentrate feeding.
Therefore, kilograms of grass drymatter produced in the spring are valued at 16c, compared to 11c in the autumn and just 4c during the summer. These values account for 31pc of the weightings in the overall index.
Silage yield is also calculated separately, again with different values attributed to first and second cuts.
First cut kilos of drymatter are factored in at 4c/kg, while second cut is discounted by 25pc to 3c/kg.
Silage yield is rated as half as important as grazing yield, and so it only accounts for 15pc of the total score of any one variety.
The next trait that the formula aims to account for is the variation in digestibility of grass at various stages throughout the year.
Falls in digestibility in April, May, June and July are costed at approximately 1c per unit change in digestibility during these months.
Mid season is the key period where grass quality can influence animal performance to a significant degree. These digestibility values constitute 20pc of a variety's overall weighting.
The remainder of a grass's score -a full 34pc- is based on its persistency, which is measured from the changes in ground cover over a 12 year modelled period.
The end result is an overall score that allows farmers to easily rank grass varieties, but Dr O'Donovan stressed that farmers keen on particular traits, such as silage yield, will be able to also rank varieties according to their sub-indexes, which include:
• Spring grazing yield
• Summer grazing yield
• Autumn grazing yield
• Silage yield
• Quality (digestibility)
Over the coming years, Dr O'Donovan plans to incorporate more traits into the index, such as animal effects.
"This is just the start - it's going to get stronger over time, just as the EBI did," he said.
"But it has already shown up large differences between the varieties listed in the Department of Agriculture's recommended list."