16 May 2018
Dr Elizabeth Hart: Farming Connect Knowledge Exchange Hub, IBERS, Aberystwyth University.
Take home messages:
- Current research aims to reduce greenhouse gas (GHG) emissions, particularly enteric methane.
- Methane production comes from a community of microbes within the rumen, which may be altered through dietary changes.
- Methane mitigation may be achieved by selective forage breeding and grazing management strategies.
Ruminants possess a complex microbiome that enables the conversion of plant carbohydrates into useable energy for the animal. A by-product of this process is the generation of enteric methane, which contributes towards global warming. Methane production depends on the microbial community within the rumen that generate methane. This microbial community has the potential to be manipulated alongside the other rumen microbiota without detrimental effects to the animal and even has the potential to benefit animal productivity. The composition of feed has been shown to influence the levels of methane produced by ruminants, therefore much research has focussed on reducing methane emissions through feeding and management approaches.
Selective forage plant breeding and pasture management
In the UK, grasslands are largely depended on for livestock production and in Europe ryegrasses account for over 60% of agricultural grass seed used. Selective forage plant breeding offers an opportunity to reduce methane emissions without compromising productivity by modifying plant composition. Optimal nutrition requires the identification of suitable forage varieties and the determination of quality traits for plant breeding programmes. A good way of improving the energy use efficiency of forages, which in turn reduces the level of methane emissions is by increasing the quality of forage. This can be achieved by harvesting or grazing less mature forages and by selecting for forages with higher digestibility. New grass varieties are available to farmers which can provide better yield and digestibility compared to older varieties. Ryegrass is the most important sown grass in the UK, producing high quality yields; however, the presence of weed grasses in pastures such as annual meadow grass can decrease the yield quality. By observing a red colouration at the base of the grass stem farmers can check for the amount of ryegrass present at pasture. Differences in quality characteristics and growing conditions alter between ryegrass varieties, which include both diploid and tetraploid plants that vary in growing conditions, digestibility and palatability. Selecting the most appropriate grass for each field is important and sowing with a mixture of varieties can be beneficial in terms of nutritional value and productivity. Intensifying the number of legumes present in grazing pasture has also shown to result in a reduction in methane emissions compared to the presence of ryegrass alone. Legumes such as white clover have a higher digestibility compared to general grasses, which enables higher intakes that contribute to lower methane yields. Ideally pasture with a mix of 30% white clover and 70% grass is a good balance but there can be much variability on these percentages. Comprehensive coverage of recommended varieties of ryegrass and clover lists selected for specific characteristics based on cutting trials are available at the British Grassland Society online database.
Research at IBERS reflects the need for high quality producing forages and for increased nutritive value by combining traits from Italian ryegrasses and hybrids to improve rumen efficiency and by providing ways to mitigate environmental concerns such as methane emissions. The quality of forage can have an effect of lowering methane yield by boosting the levels of forage intake; hence improvement of grasslands on farms to increase yield and quality has considerable potential. Research has shown that increasing the water-soluble carbohydrate (WSC) contained in fresh forages has resulted in the most readily available energy source for grazing animals and IBERS perennial ryegrass (PRG) breeding programme have produced ryegrasses with elevated concentrations of WSC, such as AberMagic, which have been shown to directly reduce methane emissions. For example, it has been demonstrated that increasing WSC in perennial ryegrass by 33g/kg results in a decrease in methane production by 9% in laboratory experiments and in field trials with PRG containing 20.5 g WSC/kg DM lambs were shown to produce 25% less methane compared to those on control grass.
Grazing management can also affect methane yield and it has been suggested that a more intensive approach to livestock production may reduce methane emissions. For example, management-intensive grazing has been shown to be a best management practice that has the potential for the more efficient use of forage crops resulting in a 22% reduction in methane emissions compared to continuous grazing systems. However, a higher stocking density must be matched with an increase in feed supply and there are other important issues to consider such as the trade-off for other environmental impacts. For example, as ruminant systems become more intensive, the environmental impact shifts from methane to nitrous oxide (N2O) production and animal welfare also becomes another concern, especially for indoor systems. For pasture management, previous research has shown that allowing swards to reach higher digestibility values has actually resulted in an increase in daily methane emissions, however due to the rise in forage intake the subsequent performance emissions per kg gain are reduced as animals reach target weights earlier. It is important to consider the lifetime performance of grazing animals as increases in growth and performance enables livestock to finish promptly and more efficiently, thereby reducing the environmental footprint.
It is evident that there are several ways to reduce enteric methane production and the above describes some measures that could be applied to grazing ruminants. However, care needs to be taken to ensure that whatever strategies are employed on farm, that animal performance and welfare are not detrimentally affected or that environmental impact shifts do not occur. Forage breeding programmes for feed quality improvement and grazing management practices hold potential not only for methane mitigation but for the overall enhancement of animal performance. With the application of such strategies provision towards the UK commitment to decrease GHG emissions for the future will be an achievable goal. Economically, initial focus on farm management practices will be important as changes made here will have a more immediate and lasting effect.