Monthly Archives: September 2013

Silvopastoral farming in Colombia. Photo: Neil Palmer, CIAT

Trees on farms increase milk yield by a factor of five

‘Silvopastoral’ farming has multiple benefits for farmers, cows and the environment

A new paper published by the Royal Society demonstrates an extraordinarily wide range of co-benefits from a relatively simple technique: silvopastoral farming (farming with trees). Planting trees on grazing land not only cuts greenhouse gas emissions but also improves animal welfare, reduces air and water pollution, avoids soil erosion, protects biodiversity and can increase the milk yield by a factor of five.

The review was produced by Professor Donald Broom at the University of Cambridge, working with co-authors from Mexico and Columbia. It challenges the conventional model of grazing, in which livestock eat only grass, by showing that introducing trees and shrubs to pasture can increase productivity.

“Excessive focus on pasture plants for the feeding of farmed ruminants has been a major mistake in almost all parts of the world,” explain the authors. By growing trees and shrubs with edible shoots and leaves, in combination with pasture plants, farmers can produce more fodder per hectare and provide a richer diet for their animals.

The greatest benefits come from growing leguminous species, which fix nitrogen from the air. This also avoids the need to apply artificial fertilisers, saving money as well as avoiding air and water pollution. Widely used leguminous “fodder trees” include various species of Leucaena – a drought-tolerant shrub that is loved by cattle and grows fast in tropical conditions. One trial showed that growing Leucaena on pasture increased the mass of fodder available to cattle by 27% and the protein content by 64%. A similar trial showed that milk yield per hectare increased by a factor of four to five, mainly because many more cattle could be kept on each hectare.

Trees and shrubs provide valuable habitat for native wildlife. “Conservation need not just involve tiny islands of natural vegetation in a barren world of agriculture, as there can be great increases in biodiversity in farmed areas,” the authors explain. Silvopastoral farms can provide wildlife corridors, connecting the remaining areas of natural habitat. And because milk yield per hectare is higher, there is less need to clear more forests for agriculture, giving further benefits for biodiversity.

Climate benefits are significant – through increasing the amount of carbon stored in the soil and vegetation and avoiding deforestation, but also through cutting emissions associated with manufacturing and applying fertilisers. Fertilisers are energy-intensive to produce, and once applied to the land they emit nitrous oxide, which is a greenhouse gas with a warming effect more than 300 times that of carbon dioxide.

Trees and shrubs also stabilise the soil, reducing soil erosion. The deep network of roots increases the ability of the soil to absorb and retain water, making the farm less vulnerable to droughts and floods. Soil fertility improves, as the tree roots bring up nutrients from deep in the ground, and recycle them to the land when leaves fall. The paper cites “islands of extra soil fertility under the canopy of trees”, and more favourable conditions for earthworms and beneficial insects such as dung beetles. And farmers gain a free source of wood to harvest for fuel or building, or for sale as an alternative source of income.

Perhaps the most surprising benefit is that the cows themselves are happier, exhibiting more settled and less aggressive behaviour. It seems that they like having trees to hide behind – it gives them a sense of security. Trees also provide shade in hot weather and shelter from wind and rain. There is even a reduction in diseases from parasites such as ticks, as the trees attract birds which are natural predators of ticks. These factors make working conditions more pleasant for the farm staff as well, and silvopastoral farms report higher retention of staff and higher job satisfaction.

Silvopastoral farming offers the tantalising prospect of genuinely sustainable high-welfare livestock production, and the authors conclude that it should be further developed.

Sustainable, efficient livestock production with high biodiversity and good welfare for animals

D A Broom, F A Galindo and E Murgueitio (2013). Proceedings of the Royal Society B, 280, 1771, November 2013. doi: 10.1098/rspb.2013.2025



awesome sky

Cleaner air from climate policy could save two million lives every year

Savings from reduced air pollution outweigh the cost of climate policy through to 2050

A study published in Nature Climate Change estimates that climate policy could save millions of lives every year by cutting air pollution. The health benefits would outweigh the cost of cutting greenhouse gas emissions.

The research team, led by Dr Jason West at the University of North Carolina, used the latest data on the health impacts of fine particle and ozone pollution to model two scenarios: one with strong global action to limit greenhouse gas concentrations to 525ppm by 2100, and one with no climate action. The climate action scenario entailed a shift away from using fossil fuels, which are the main source of air pollution, by cutting energy demand and switching to nuclear and renewable energy (mainly wind) and biofuels.

They found that the scenario with strong climate action led to half a million fewer premature deaths from air pollution globally every year by 2030, rising to 1.3 million in 2050 and over 2 million by 2100. The model included the impacts of fine particles on lung cancer and cardiopulmonary disease, and the effect of ozone on respiratory problems.

The health benefits are worth between $50 and $380 for every ton of carbon dioxide removed – which is more than the cost of cutting carbon emissions in 2030 and 2050, and within the lower part of the range of cost estimates in 2100.

The health benefits were strongest in the densely populated regions of East and South Asia, North America and Europe, especially in highly polluted parts of Asia. In 2030, two thirds of the global benefits occurred in China, and in East Asia the benefits were 10 to 70 times greater than the costs of cutting greenhouse gas emissions.

By linking a model of the global atmosphere with a model of energy economics, the team were able to account for a number of factors not included in earlier assessments. They included the effect of long range transport of pollution, the growth in ozone pollution due to methane emissions and the impact of climate change itself on air pollution (such as increased ozone production in hotter, sunnier conditions). They also used a higher monetary value for the loss of a human life in future years, as a result of economic growth, and took account of increasing population and susceptibility to pollution. Together with the use of updated values for the health impacts of air pollution, this resulted in higher estimates of co-benefits than those produced by previous studies.

However, the study did not include the health benefits for children and adults under the age of 30, the benefit of avoided illness (rather than premature death) due to air pollution, or the effect of climate policy on indoor air pollution from stoves in developing countries. Inclusion of these effects could result in even higher estimates of the benefits of climate policy for human health.

Co-benefits of mitigating global greenhouse gas emissions for future air quality and human health (2013) J. Jason West, Steven J. Smith, Raquel A. Silva, et al., Nature Climate Change 3, 885–889 doi:10.1038/nclimate2009