Summary: probably the most feasible strategy to reduce the most suffering on Earth, with currently available technologies, is reducing the consumption of products from small animals such as chickens, fish and shrimp by replacing those products with plant-based meat alternatives, and replacing the vacant cropland (that was used for animal feed) into natural habitat, especially grassland with large herbivores, especially horses whose populations are controlled with immunocontraception.

It is not easy to effectively reduce a lot of suffering: due to the ‘harm cascade’, measures to reduce suffering often have negative side-effects that increase suffering. Here I present a strategy that most likely is able to reduce huge amounts of suffering with currently available knowledge, means and technologies. 

First, we look for places on earth that have the most dense suffering, i.e. the most suffering per unit surface area. These are probably places of intensive farming of small animals: chicken farming (broiler chickens and laying hens), aquaculture (small fish and shrimp) and insect farming. Most people believe that most farmed animals have a negative welfare, and in the case of a chicken, they believe that the negative welfare is equal in size to the happiness of a human. As a result, most people would come to the conclusion that the expansion of small animal farming is the reason why net global welfare on earth is most likely negative and declining. 

So as a first step, we can reduce the consumption of products from small animals, such as chicken meat, eggs, fish and shrimp. If people would replace chicken meat and eggs by beef and dairy products, that would reduce farmed animal suffering, because the welfare footprint (the moral footprint and deathprint) of chicken meat and eggs is an order of magnitude larger than the welfare footprint of beef and cheese. But even better would be replacing animal-based products with animal-free (plant-based) alternatives. That became feasible due to new food technologies, especially for chicken meat. In the largest blind taste test, plant-based nuggets are the first animal-free product that outperformed an animal-based meat equivalent. As the production of plant-based meat requires less resources (land, labor) than chicken meat, we can expect that those plant-based no-chicken nuggets become at least as cheap as chicken nuggets (especially after they profit from economies of scale and agricultural subsidies for chicken meat are abolished or the climate change, public health risks and animal suffering external costs of chicken meat are included in the price). 

Reducing the consumption of chicken meat and replacing it with plant-based meat, not only reduces chicken suffering, but has a few co-benefits: it reduces the risk of a bird flu pandemic and reduces antibiotic use, air pollution and climate change.

However, the production of animal-free, plant-based protein requires less land than chicken meat and eggs. Hence, we have to consider what will be done with the agricultural land that becomes available when we switch from chicken farming to plant-based food production. There are five options.

First, the freed up land can be used for extra infrastructure for humans. Cropland used for animal feed involves wild animal suffering (due to tillage, harvesting and pest control). The total suffering per unit area on cropland used for animal feed production is larger than the total suffering on built-up land, so replacing cropland with infrastructure would decrease suffering. But it is unlikely that so much cropland will be turned into built-up land. 

Second, the land can be used to grow energy crops instead of animal feed crops. That cropland will still involve wild animal suffering, but the energy crops to produce biofuels can replace fossil fuels that contribute to climate change. Hence, total suffering will not increase but will probably decrease due to less climate change. 

Third, the cropland could be turned into grazing land for the farming of ruminant animals, to produce beef and dairy. The animal suffering per unit area of grazing land is probably less than that of cropland (no tillage, less pest control). However, those ruminants contribute more to climate change due to their methane emissions, and using those animals as merely a means against their will, involves animal rights violations. Given the increasing concerns of climate change and biodiversity loss, it is better and more realistic to expect that the released cropland will be turned into natural habitat. There are two types of natural habitat to consider, forests and grasslands, which brings us to the final two options.

Fourth, the cropland is turned into forests. It is not clear whether forests have higher or lower levels of suffering compared to cropland. On the one hand, forests are not faced with highly invasive procedures that cause animal suffering, such as tillage, harvesting and pest control. Forests can also absorb more CO2 than cropland and hence help mitigate climate change, resulting in less suffering.  On the other hand, forests have four layers (the forest floor, the understory, the canopy and the emergent layer), whereas cropland has only two layers (the soil and the aboveground crops). That means forests contain more animals (including insects and other invertebrates) than croplands, and that could mean more animals are suffering in forests.

Fifth, the cropland is turned into grassland with large herbivores (e.g. horses). This is probably the best alternative for the released cropland. For three reasons.

1. The grassland contains fewer animals than forests, because the grassland has, like cropland, two layers instead of four. But grassland with large herbivores probably has fewer animals than cropland as well, because the cropland can have many, small herbivores (e.g. rabbits) and omnivores (e.g. rodents). A large herbivore consumes more of the net primary production than a small herbivore. Large herbivores on grassland eat more plant material per animal than small herbivores in forests or croplands. This means ecosystems cannot sustain large numbers of animals if it has large herbivores. And fewer animals means less animal suffering. Given the uncertainty about the welfare levels of wild animals, and given the moral preferences of many people, many people prefer a world with fewer animals that are more likely to have positive lives above a world with more animals that could have very negative lives. According to a survey, most people estimate that the welfare of an average bird in nature is close to zero, but it can equally be positive or negative. Many people favor an asymmetric population ethical theory, which says that bringing animals into existence that have a negative welfare is very bad, whereas not bringing animals into existence that have a positive welfare is not bad or less bad. Many people favor avoiding suffering above creating happiness. Many people are risk averse and prefer a world where an animal does not exist (and hence has a zero welfare) above a world where there is a 50% probability that the animal has a positive welfare and 50% probability of the animal having an equal sized negative welfare. 
2. The suffering of a few large herbivores can more easily be mitigated than the suffering of many small animals. Large herbivores face predation only by large predators, and it is easier to decrease and avoid predation by large predators than by small predators. In many areas (e.g. western Europe), there are almost no large predators such as wolves that can kill large herbivores. When in those areas cropland that is currently used for chicken feed is turned into grassland with horses, all it takes for those horses to avoid predation is to prevent the reintroduction of large predators. Preventing such reintroduction is feasible. With immunocontraception it is also feasible to decrease the fertility of large predators and thereby decrease the predation pressure on the herbivores. To avoid overpopulation of large herbivores in the absence of predation, it is also feasible to use wildlife fertility control methods, especially immunocontraception. Especially for horses, there is a humane and cost-effective immunocontraception already in practice: PZP. Instead of killing animals, hunters can assist in wildlife fertility control of horses by shooting PZP-darts to mares. 
3. Compared to cropland and built-up land, grassland can absorb more CO2 and hence mitigate climate change. Compared to ruminant animals used for livestock farming on grassland, horses (and elephants and kangaroos) have lower methane emissions. Hence, grassland with horses is better for the climate than grassland with cows or sheep.  

More free-roaming large herbivores on grassland close to urban areas can have a negative side-effect: more car accidents. This can be mitigated with fences and wildlife crossings. And before we have so much large herbivores crossing our streets, we will have self-driving cars that are better able than human drivers to avoid collisions with those animals.

A bit simplified, we can summarize the above by saying we replace a large number of disease-prone broiler chickens and laying hens in dire, crowded conditions with a much smaller number of healthy, free-roaming horses. We also need to do more research on the welfare of wild animals. If it turns out that the welfare of animals in a forest is sufficiently higher than the welfare of the animals on the grassland with horses, we can turn that grassland into forests.