Why cattle ranchers keep trees on pastures
Recent research comparing semi-natural grasslands and sown pastures in Nicaragua reveals that their productivity falls off in the dry season and then completely peters out. This explains the widespread practice of retaining trees and shrubs on pastures. Trees, with their deep root systems, may produce green leaves and nutritious pods or fruits even at the driest times of the year.
The research, published by Sonia Ospina and her fellow researchers in the open-access journal PLOS One, was initiated to explore how the widespread practice of converting semi-natural grassland to sown pasture, affects productivity. Fergus Sinclair, Research Leader in Production Ecology at the World Agroforestry Centre, collaborated on the work and explains: “Farmers and rural advisers assume that replacing semi-natural grassland with sown pastures will result in more feed for their cattle but we found, when you track productivity throughout the whole year, the higher productivity of sown pastures in the rainy season is offset by lower productivity in the dry season”.
As Ospina points out, “sown pastures are dominated by a single erect grass species, Brachiaria brizantha, and tail off in productivity more quickly when it gets dry, whereas some species in the more diverse grasslands are more tolerant of dry conditions and maintain their productivity for longer into the dry season. The grasslands are comprised of many prostrate grass species mainly in the Paspalum and Axonopus genera. More stable productivity can be expected with higher diversity because diverse communities are likely to include combinations of species that are functionally different from one another – such as species with different rooting patterns and phenology.”
“What this means for farmers, is that productivity is more even throughout the year for grasslands than sown pastures” emphasizes Sinclair. “The grasslands are like the tortoise rather than the hare in the eponymous race - slower but steadier in their production. Although we have not compared their nutritional quality, this may also be different.”
“While farmers could conserve surplus production from pastures in the rainy season to use in the dry season by making hay or silage, that would involve a lot of labour and other costs. To get the most from sown pastures farmers would need to fertilise and irrigate, but that is not common in the area. So, all in all, what our research shows is that the widespread replacement of grassland by sown pasture is of questionable value in terms of improving the quantity of feed available for cattle. But, neither sown pastures nor grasslands, are productive at the end of dry season, which explains why farmers typically retain trees and shrubs in their pastures that may produce leaves, pods or fruit that cattle can eat when there is very little feed available from the herbaceous plant layer.”
Graciela Rusch, a researcher involved in the study from the Norwegian Institute for Nature Research, stressed the implications of the results for understanding resilience to climate change. “According to climate models” she said, “rainfall is forecast to decrease over the next 100 years in Central America by 10% in the dry season and 20% in the wet season. This makes it especially important for us to understand the response of productivity of different pasture types to different rainfall patterns. Our research is unique in establishing relationships between pasture productivity and the amount of rainfall over short, three-week periods. These relationships are quadratic rather than linear, which means that productivity increases with rainfall up to a threshold amount of rain and then declines. We have found that sown pastures are more sensitive to rainfall than grasslands, which indicate that they might be less resilient to climate change.”
Sonia Ospina did her research as part of a joint doctoral programme between Bangor University in the UK and CATIE, a regional centre for tropical agricultural research and higher education in Central America. “It is rare for people to measure productivity” says Sinclair, “because it requires painstaking work and repeated measurements through time. More often, inferences about productivity are made from measuring standing biomass. Sonia deserves accolades for her commitment and attention to detail.”
Recently CATIE, based in Costa Rica, began a collaborative arrangement with the World Agroforestry Centre, headquartered in Nairobi, Kenya to strengthen agroforestry research in Latin America. Dr Ospina recently joined the Colombian agricultural research service (Corpoica) as an agroforestry researcher at their Motilonia research station in Cesar.
Full research article:
Harvey, C.A., Villanueva, C., Esquivel, H., Gomez, R., Ibrahim, M., Lopez, M., Martinez, J., Munoz, D., Restrepo, C., Saenz, J., Villacis, J. and Sinclair F.L. (2011). Conservation value of dispersed tree cover threatened by pasture management. Forest Eco.