On Thursday 1 September, Hugh Harbison will host a farm walk as part of the ArcZero project on his dairy unit at Aghadowey, Co Derry.

The event will demonstrate how high output livestock farming can be managed to deliver positive outcomes for environmental sustainability.

Hugh farms with his father Thompson, running 180 autumn cows on approximately 100ha of grassland, two-thirds of which is owned with the remainder being rented.

Cows are crossbred using Holstein, Scandinavian Red and Jersey genetics. The herd is calved in a 12-week block from early September with a large percentage of cows calving outside.

Fertility is excellent, with 73% of cows settled in-calf after six weeks of breeding with an empty rate across the whole herd at 12%.

Milk yield is typically 8,800l per annum from 2.5t/cow of concentrate fed, giving approximately 3,300l of milk from forage, a large percentage of which is produced from grazed grass. Rolling average butterfat is 4.4% with 3.53% protein.

Carbon audit

As part of the ArcZero project, a carbon audit was carried out in 2021 to establish a baseline for the farm using the Agrecalc programme from the Scottish Rural College (SRUC).

Gross emissions totalled 2,020t of CO2 equivalents, or 1.2kg CO2e per litre of milk produced. However, gross emissions do not account for carbon sequestered on-farm through soils, hedgerows and woodland.

The audit estimates that soils on-farm are sequestering 525t of CO2e annually with an additional 24t captured by trees and hedges. Soils were sampled to a depth of 30cm to measure carbon stores.

When deducted from gross emissions, the farm has a net figure of 1,470t CO2e or 0.89kg CO2e per litre of milk produced.

A LiDAR survey was also carried out last year and measured the physical tonnage of carbon being stored above and below ground. Soils are holding 18,549t of carbon with a further 538t locked up in hedgerows and trees.

An interesting element of the LiDAR survey showed there was 18.78km of hedgerows on-farm, measuring anywhere from 4m to 10m in height.

Within this figure, 3.89km of hedges measuring 10m in height is storing 229t of carbon, four times more than 10.3km of hedges measuring 4m high.


Around 40% of the greenhouse gases produced on-farm are generated through enteric fermentation, which is ultimately methane gas produced by the cow’s digestive system.

Purchased feed accounts for 21% of emissions with 17% coming from the manner in which organic manures produced by livestock are stored and applied to grassland.

Multi-species swards

Like many livestock farmers, Hugh has introduced multi-species swards (MSS) on-farm in an attempt to reduce the tonnage of chemical fertiliser used on farm annually.

A 5ac paddock was sown out in August 2021, with seed drilled following the old sward being burned off.

The paddock was deliberately selected as the LiDAR survey indicated there was a higher risk of nutrient run-off within the field.

Therefore, as no fertiliser would be applied to the MSS along with the deeper rooting nature of the herbs and legumes, the risk was greatly reduced.

The MSS has performed beyond expectation in 2022. Grass grown from April to the end of July was 8t DM/ha, 3t DM/ha more than the previous sward over the same time period last year.

The sward is part of the grazing platform and has been grazed five times this year on a 21 to 24 day rotation. Four dressings of slurry have been applied to the sward this year using a trailing shoe.

The performance of the MSS has given Hugh the confidence to increase the area sown out to more diverse swards.

A further 10ac across two grazing paddocks were burned off in late July and drilled with the same six species mix in early August. Clover has also been stitched into other grazing paddocks as an alternate nitrogen source.

Reducing cow size

Hugh has also adapted his breeding programme, with the aim of producing a smaller cow without losing the output.

Not only will this ease the pressure for grass on the grazing platform, a lighter cow will be easier to graze and possibly have an extended grazing season as a result.

As the primary source of emissions on-farm, there is also a potential 10% reduction in CO2e produced annually.

Hugh maintains that a 600kg cow that can produce 600kg of milk solids will be a more profitable animal in his herd than a 700kg animal doing 600kg of solids annually.

The move towards reducing cow size is not being taken on a whim. Hugh has done his sums based on the Agrecalc programme, with a target of 500kg to 550kg liveweight.

Even allowing for a 5% drop in milk yield as cow size is reduced, cows would consume 2kg/day less concentrate and increase margins over feed costs by more than £15,000 at current ration prices.

Farm walk

  • 180 cows milked.

    100ha grassland unit.

    Annual yield 8,800l/cow on 2.5t concentrate.

    3,300l/cow from forage.

    4.44% butterfat, 3.53% protein (722kg milk solids/cow).

    ArcZero demonstration farm.

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