Livestock production needs to take into account that natural nitrogen and phosphorus cycles can be disrupted and lead to environmental damage, involving pollution of waterways, acidification, eutrophication, and biological damage. This is of course also valid for crop agriculture. Well-managed animal husbandry systems focus on improved feed strategies, fencing, and appropriate manure handling. Systems that improve soil health through regenerative approaches also have the potential to increase water infiltration and retention, thus reducing runoff into waterways.
Unsustainable practices of animal husbandry and feed production can negatively affect the natural nitrogen and phosphorus cycles, releasing excess nitrogen and phosphorus into ecosystems where they can lead to acidification, eutrophication, and biological damage [Pelletier & Tyedmers 2010; Nordhagen et al. 2020]. In ecosystem management, it is especially important to protect waterways
from contamination by animal manure and the
fertilizers used for feed production. Leaching of manure can
additionally also contaminate the water with pathogens and antibiotic
residues [Tilman et al al. 2002; Nordhagen et al. 2020].
In the EU, for instance, agriculture's total loss of reactive nitrogen in air and water amounts to 7-8 million tonnes/y (80% of total losses), mainly as ammonia (from stables and manure storage and spreading), nitrous oxide (from fertilizers and fossil fuel use), and nitrates (from manure leaching). When accounting for feed, the losses from livestock represent >80% of all agriculture. Plant-based foods have lower nitrogen losses per unit of protein compared to AFSs (25x for beef and 4-8x for monogastric meat, eggs, and dairy, when compared to cereals) [Westhoek et al. 2016]. That being said, the solution is not so simple as the usual narrative ('a shift from livestock to plants') seems to suggest, for a number of reasons outlined below.
1. Beware of generalization: outcomes depend on context
Reactive
nitrogen losses vary widely among individual production systems, as a
function of climate and rainfall, soil type,
fertilizer use, livestock housing, and manure storage type. As an example,
regional means per kg of beef carcass weight in the US range from 100 g N in the
arid Southwest (driven by ammonia) to
300 g N in the wetter Southeast, where more fertilizer is used (driven by nitrate leaching) [Rotz et al. 2019].
Of course, there are also important differences between industrialized
systems of problematic high-nutrient input and to the undersupply in
low- and middle-income countries [Paul et al. 2021].
With respect to soil type, nitrate leaching is typically higher in poorly structured sandy soils than in finer-textured ones, such as clay. This will also depend on the abundance of macropores created by earthworms, plant roots, or wetting/drying events [Edwards 2017]. Effects depend upon the type of flow and the location of nitrate throughout the soil profile. 'Preferential flow' of soil solution at high speed leads to higher leaching, whereas water in 'bypass flow' moves
down the macropores without distribution through the soil matrix,
leaving the nitrate that is contained within aggregates undisturbed.
2. A shift to more plants would come with its own trade-offs
A common (yet simplistic) message at policy level is that halving the production of ASFs would lead to major reductions of agricultural reactive nitrogen losses (around 40% in the EU) [Westhoek et al. 2016]. Not only does it downplay the full impact of such drastic food system intervention on public health, culture, and livelihoods [see elsewhere], it also may underestimate the environmental trade-offs caused by a shift to more plants. Also, the impact of ultra-processed foods is hardly ever addressed even if these foods account for a large proportion of fertiliser use, while being nutritionally unnecessary and likely harmful for public and planetary health [Blair & Sobal 2006; Anastasiou et al. 2020]. While most of the nitrogen use in the Swedish diet was related to feed production, phosphorous use was mostly ascribed (42%) to the production of sweets, snacks, and drinks (excluding dairy) [Moberg et al. 2020].
Ecotoxicity, eutrophication, and acidification can as well occur with plant production, while much depends on the type of plant and the practices involved [Frankowska et al. 2019]. In Brittany, for instance, nitrate levels of surface and groundwaters have decreased despite the high density of livestock, whereas increases are seen in specialized crop areas (beyond the French limit of 50 mg/l) [Peyraud & MacLeod 2020]. Arguing for a reduction of livestock farming will likely lead to an expansion of arable and horticulture farming, which may worsen some of the concerns as they usually have higher leaching rates of nitrogen on a per hectare basis.
In New Zealand, for instance, values for ruminant livestock are estimated at 10-60 kg N/ha/y, while horticulture systems lead to values up to 300 kg N/ha/y [Edwards 2017]. The reason is that the latter are usually characterized by high fertilizer input, intensive cultivation, and short periods of plant growth. Regular cultivation causes an increase in mineralization, while the fact that fields are left bare for periods of time implies an absence of plant uptake. Often only 20-50% of the added N is utilized. Although arable and horticulture systems are similar in terms of leaching, the latter use higher rates of nitrogen fertilizer (up to 600-900 kg N/ha) and depend on plants with shallow root systems that lead to a lower nutrient uptake and a larger leaching of non-utilised nitrate.
Finally, the
elimination of livestock, rather than the improvement of the
integration of animal and crop agriculture, would make the food supply
system increasingly dependent on synthetic fertilizers produced from
fossil fuels. It is not fully clear if organic or synthetic fertilizers are better or worse for greenhouse gas emissions [Walling & Vaneeckhaute], but it could be that the problem for nitrous oxide from livestock has been overestimated [AgResearch 2020]. Be that as it may, the use of synthetic N fertilizers is a main driving force of increasing nitrous oxide in the atmosphere [Smith 2017].
Although it is fair to say that manure handling is often creating harm, it should not be forgotten that it is primarily creating organic carbon, microbial diversity, and fertility in soils, with nitrogen, phosphorus, and potassium being among the key limiting nutrients [Ye et al. 2020; Nordhagen et al. 2020]. Manure thereby acts as a better fertilizer for increasing crop yields than synthetic fertilizers do [Cai et al. 2019].
3. Nutritional value affects in-between food comparisons4. Further mitigation of harmful livestock effects are possible
Without question, and despite the nuance offered above, many outcomes of livestock farming are harmful with respect to their disruptive effects on nutrient cycles and on water pollution. These aspects can and should be addressed where such issues arise, and a lot of potential is available indeed. Mitigation options include improved feed strategies and precision feeding, fencing, fine-tuning of irrigation, as well as manure treatment and management [Smith 2017; Peyraud & MacLeod 2020; Dybowksi et al. 2020].
Herd management and adaptive multi-paddock grazing
Herd management offers additional possibilities to rebalance nutrient cycles, especially for ruminants. Grasslands have high rates of biogeochemical cycling, using water resources more rapidly than other ecosystems. Its herbivores populations eat and trample the vegetation produced during the rainy season and return nutrients to the soil through their manure [Zimov 2005]. Well-managed livestock, especially cattle mobilized using multi-paddock strategies that mimic nature, will improve soil health, biodiversity, and forage biomass [see elsewhere], while increasing water infiltration and retention, and reducing runoff into waterways [Weber et al. 2011; Park et al. 2017; Hillenbrand et al. 2019; Pérez-Gutiérrez & Kumar 2019; Dowhower et al. 2020]. Although mostly beneficial, results are not always equally consistent, once more suggesting context dependency [Kleppel 2020].
For a science compendium on well-managed grazing, see: Soil4Climate 2023
Explanatory video 💬 Williams 2024
