Prelude: Methane emissions in particular, and nitrous oxide as potent greenhouse gases (GHG), have been in the political, activist and general public debate for some time. Central in the debate has been livestock ruminants, in particular beef and dairy cattle, since estimates show the global production of these gases from livestock may be upwards of 20% of total. Since methane is a comparatively short-lived polluter in the atmosphere and perceived to have a high global warming potential, it is logical that the focus to reduce emissions should be on this gas, and therefore on beef and dairy cattle where methane is produced by enteric fermentation in the rumen, with some additional emissions also coming from their manure. Therefore, on the global environment protection platforms, supported by anti-animal activism, the call to decrease cattle numbers has increased. In fact, in the EU, several countries have promulgated legislation to reduce cattle numbers by up to 30% by 2030. Nobody questions the necessity of reducing methane emissions, but the question is if the policy to reduce cattle numbers is correct. This review summarizes ways to reduce GHG and endeavour to answer the question whether reduction in cattle numbers is the way to go.
Reduction methodologies: Methane: Enteric methane production can be reduced by feed composition and processing manipulation, feed additives, increased productivity and genetic means. In feed composition, less methane is produced in the rumen per kg feed with high quality forages and concentrates than with low quality forages with further reductions possible with feed processing.
A number of feed additives has shown potential to reduce methane production, including addition of microbial populations that increase lactate and succinate in rumen as fermentation intermediates, nitrate, 3-Nitrooxypropanol (3-NOP), ionophores such as monensin, and red seaweed. Short-term experiments have shown decreases of up to 30%, but because of adaptation of the rumen population over time, the observed reduction may not last. Nitrate, although comparatively effective, is reduced to nitrite which could be toxic to the animal, and red seaweed with active compound bromoform results in bromium in the milk as well as iodine from some species. A further limitation is that, for ecologically reasons, red seaweed has limited capacity in application when harvested from the sea and therefore has to be produced on land with cost implications. Overall, decreased methane production by feed additives should not reduce milk production and should coincide with maintained or improved feed efficiency if the methodology is to be accepted at farm level.
Perhaps the lasting long-term solution to methane mitigation is utilising genetic variability. A recent study has shown a difference in lactating Holstein phenotypes in inherent enteric methane production of 20-25% which coincided with increased efficiency. Genetic selection and associated management improvement should also emphasise higher milk yield per cow and less dry cows and non-productive heifers in the herd, as this will result in the same or more milk production with a smaller herd and consequently lower overall methane production.
Nitrous oxide: The relationship between cattle and nitrous oxide is primarily indirect through their feed supply. Nitrous oxide is emitted in comparatively small quantities from cattle manure and mainly following chemical N fertilisation and pesticide application of crops, cover crops and cultivated pastures. It is a dangerous atmospheric polluter with implications to the ozone layer and much higher global warming potential than methane and carbon dioxide, and therefore has to be addressed urgently. The implications are that agricultural practices need to limit N fertilisation and other chemical substance applications. One way is the supply of organic N through cattle manure as substitute for chemical nitrogen fertilisation by following regenerative agricultural practices.
Carbon sequestration and cattle numbers: Methane entering the atmosphere is part of the biogenic carbon pathway which results in photosynthesis. Being a short-lived gas in the atmosphere, methane is oxidised after about 10-12 years to carbon dioxide of which the carbon will be built into plant carbohydrates and through the plant roots will enter the soil where carbon may accumulate. If this process is maximised, about 2-3 times more atmospheric carbon can be sequestered than what methane emission reduction can achieve. Herbivores (later mainly cattle), have been an essential part of the biogenic carbon pathway for millennia since grazing stimulates photosynthesis, which implies that effective grazing management to maximise photosynthesis and plant biomass should be exploited in mitigation. To maximise this option, cattle numbers should not be reduced.
The question may be asked why then do we see methane accumulation in the atmosphere. If herbivores (cattle) are the only source of methane, and the numbers stay the same the emission of methane will be in equilibrium with the oxidation to carbon dioxide which during the timeframe of the biogenic carbon cycle will return to the earth, and no net warming due to methane will result. The problem largely relates to the entering of methane into the atmosphere from fossil fuel origin, peat and methane trapped beneath ice masses in the polar regions (and now being released due to global warming) is ‘new’ to the atmosphere and results in accumulation because the rate of removal is exceeded.
Other implications of reducing cattle numbers: Apart from the implications to carbon sequestration, the decision to reduce the number of cattle must always be taken in consideration of other socio-economic implications, a major one being food security – particularly in poorer countries. Beef and dairy cattle are responsible for a major part of food and sustenance which (i) cannot be replaced by plant food sources, both nutrient and volume required wise, and (ii) animal foods are necessary in many dry countries which do not have the water to irrigate plant crops.
In a study in the US where water, soils and climate are conducive to major increases in plant food production, reduction in cattle numbers to target showed that recommended nutrient levels to support a healthy nation cannot be met. Another consequence is that the targeted reduction will reduce the US overall carbon footprint by no more than 2-3%. Similarly, in SA if cattle numbers are reduced by 25% the mitigation will be of the same order, but the implications to socio-economics, sustenance and food security will be devastating. The implication is that if much larger GHG emitting sectors such as energy and transport do not simultaneously reduce their carbon footprint, the effort in the livestock sector will be fruitless.
Bottom line: The arguments above indicate that there is no justification in global reduction of cattle, but effective mitigation of methane and nitrous oxide (and of course carbon dioxide) with measures discussed above should be enhanced. This is the responsibility of all sectors of the economy. In addition, the phasing out of fossil energy and being replaced by ‘green’ energy sources should happen earlier rather than later.