A carbon audit is a useful tool to help monitor the technical efficiencies of a dairy enterprise. The carbon footprint shows the level of greenhouse gases being produced through activities on your farm. Areas of high emissions can suggest room for improvement in technical efficiency, which can be highlighted through benchmarking using industry key performance indicators (KPI’s) e.g., milk from forage and concentrate use per litre. Consequently, improving on-farm efficiencies strongly correlates with reduced production costs and emissions per kg of fat and protein corrected (FPC) milk. As forage is the main constituent of the dairy cow’s diet, and its quality is largely controllable, farmers should consider the following management factors when producing forage to reduce the emissions associated with the dairy enterprise.
Reduce fibre in diet
The composition of the diet greatly influences the emissions generated per litre of milk. In terms of promoting an efficient rumen environment compatible with reducing carbon emissions, high fibre diets are undesirable. They promote more methane-producing bacteria compared to starch-based diets, which is supported by the work of Hammond et al. (2016) who found that methane emissions were significantly greater when high fibre diets were fed.
Fibre, which is measured through neutral detergent fibre (NDF), is the main driver influencing the digestibility of forage, and relates to the fraction of hemicellulose, cellulose, and lignin within plant material. These fractions are not readily fermentable in the rumen, and in turn reduce the rate of passage and reduce intake potential. Research suggested that a diet of 32% NDF compared to 40% NDF produced 5% less methane. Due to the role of forage in promoting rumen health it is not advisable to reduce NDF below 25%. Moreover, as forage is the cheapest feed source on farm it is cost effective for it to be maximised in dairy diets. Therefore, the aim should be to improve forage quality.
Increasing forage digestibility and subsequently intake will reduce greenhouse gases associated with enteric fermentation, as well as increase milk output and therefore there will be more kgs of milk to spread emissions over, reducing the carbon footprint. The NDF fraction of the plant increases exponentially when plants mature. Consequently, to reduce the NDF level in grass silage, it should be cut prior to maturity, ideally just as the grass is heading, if not before.
Efficient use of inorganic fertiliser
The carbon footprint of the forage crop is also influenced by the technique, quantity and conditions in which slurry is applied to the land. In less-than-optimal spreading conditions (mild and windy), approximately 80% of the total ammonia can be lost when applied with a splash plate. Conversely a 60% reduction in ammonia loss can be achieved when using a trailing shoe compared to a splash plate, and between 70-90% when applied with a slurry injection system (Edwards, 2022).
If slurry is being spread onto bare ground, farmers should look to incorporate slurry within 12 hours to minimise ammonia losses, which can be seen in figure 1. Maximising the utilisation of nitrogen through low emission spreading will reduce the need for further application and inorganic fertiliser application and reduce the carbon footprint associated with the crop. Covering slurry stores will help keep rainwater out, thus reducing storage requirements and spreading costs, and also increases slurry nitrogen value.
Grass ley age and varieties
To ensure grasses are both high yielding and high quality, farmers should consider how frequently they reseed. Research by AHDB states that a two-year-old ley yielding 13.5t/ha at 12ME will drop to 11.2t/ha and 11.5ME by year five, and by 10 years be at 10.8ME.
Reseeds should contain clover - a 30% white clover cover can fix 180kg N/ha and reduce the reliance of both inorganic and organic fertiliser. In terms of animal performance, research found an increase of 7.5% in milk solid yield when feeding a grass clover ley as opposed to grass only (McCarthy et al., not dated).
References available upon request.
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