Although whole farm stocking rates don’t impact nitrate leaching, hotspots are a contributing factor.
This is according to Eddie Burgess from the Agricultural Catchments Programme (ACP) at Teagasc, who spoke at the Water Quality Conference in Tullamore, Co. Offaly.
He outlined how hotspots and the management of nutrients on farms will have an impact on nitrate leaching, and as a result will negatively effect water quality.
He explained how data collected from the ACP shows that whole farm stocking, or an increased stocking rate in a sub-catchment has little impact on nitrates leaching.
Burgess explained that within the programme, in addition to taking samples from the outlet of rivers in the programme every 10 minutes, up to four of the catchment grab samples are taken up stream every month, and sub-catchment areas can then be looked at.
From these samples, the nitrate concentration can be compared with the organic nitrogen (N) stocking rate in these sub-catchments areas.
He explained that since this has been taking place for 15 years, soil type and even weather can be taken into consideration.
However, there is a strong relationship with soil type, with free draining soil more at risk for nitrates loss, but with heavier high clay content soils, there is not.
The proportion of land being cultivated is closely related to nitrate loss, according to Burgess.
He said that data from two catchments showed that in one area where the organic stocking rate increased, the nitrate leaching did increase, but only slightly, and in another catchment area as the organic stocking rate increased – nitrate leaching actually decreased.
Hotspots
Using a hypothetical situation, Burgess outlined how on a dairy farm with two blocks of land, the main block is 50ha – with the housing and grazing of the herd taking place there.
An out block of 30ha is also used, two cuts of silage are harvested and replacement heifers are grazed there.
He explained that in this situation, the majority of the nutrient was being spread on this home block.
The whole farm stocking rate is 200kg of organic (N)/ha, but the main block is actually stocked at 310kg/ha, while the out block is stocked at 30kg/ha.
Burgess said that averages can hide hotspots: “If we want to tackle N loss, we have to look at hot spots – they can occur for many different reasons.”
He said that there is no silver bullet, but a good understanding of the N cycle in important.
Burgess said we shouldn’t ignore the hydromorphology of the landscape and timing of actions are critical.
He advised those attending the conference to target hot spots and reduce the nutrient load on these areas.