A case study of greenhouse gas emissions on deer farms has shown only small reductions in emissions can be achieved by making management changes short of reducing the head count.
The study by environmental agribusiness consultant Alesha Cooper from AgFirst Manawatu-Whanganui was of four farms selected by Deer Industry New Zealand to represent typical deer farms.
Two North Island hill country farms, one South Island high country and one South Island flat to rolling country were studied.
Two of the farms were breeder-finishers, one velvet-focused and the other a venison finisher. Carbon emissions were estimated using Overseer FM.
Farms with land suitable for other uses such as carbon, production forestry or cash cropping were likely to have the greatest ability to reduce their emissions.
Coper said converting parts of a farm to forestry is one of the few ways farmers can significantly reduce emissions.
All four farms were running deer, cattle and sheep at ratios ranging from 22% to 79% deer.
In all cases sheep had lower carbon emissions per stock unit than deer or cattle, which had similar emission levels.
The study found the effectiveness of mitigation options depends on the farm and might include increased per animal performance and lower stocking rate maintaining or improve profitability while reducing drymatter intake and therefore methane emissions.
Reducing nitrogen intake by using low-nitrogen feeds such as grains and fodder beet needs to be a major part of the diet to have much effect.
Increasing the ratio of lower-emitting stock classes, for example by going from 40% sheep to 60% sheep and reducing the number of breeding animals by replacing breeding cows with finishing bulls will increase feed efficiency by putting more feed to production rather than herd maintenance.
“These and other options were assessed on the sample farms but even if several mitigations were adopted and the resulting emissions reductions added together the net benefit was unlikely to be more than 5%,” Cooper said.
The most significant single reduction modelled was 5.7% achieved by increasing the lambing percentage on one of the farms by 20%, which allowed a reduction in ewe numbers.
Cooper said much is made of the inability of farmers to offset methane emissions with carbon forests, including shelter belts.
If that rule is changed and offsets are allowed in trees planted since 1989, the offsets earned by the four case study farms from existing plantings would range from zero to 46.5%.
But forestry is not a permanent solution for offsetting gas emissions because more trees would have to be planted after every harvest.
“If we assume 100ha of radiata pine forestry is sufficient for offsetting emissions on a farm, after 28 years the forest would reach maturity and need to be harvested.
“At that time the initial 100ha would need to be replanted to offset the carbon removed at harvesting, plus an additional 100ha would need to be planted to offset continuing emissions.
“At the second harvest, the 200ha would need to be replanted as well as an additional 100ha and so on,” Cooper said.
“If you are considering forestry for carbon sequestration and offsetting it is important to get good advice.”
Cooper highlighted a number of mitigation options might become available in future including methane and nitrification inhibitors.
A methane inhibitor for deer would need to be delivered by way of a slow-release bolus or another technology that does not require animals to be frequently handled.
Nitrification inhibitors have been shown to reduce nitrous oxide emissions but they can’t be used on NZ farms because there is no internationally accepted tolerance for their use on feeds eaten by food animals.
Methane emissions also vary from animal to animal, a difference that is thought to have a genetic basis, so selecting for low-emitters might one day be possible.
To read the study summary report and individual farm reports go to www.deernz.org/deerhub/farm-environment/climate-change/action-reduce-emissions