New paper quantifies the global role of livestock as a nutrient source for the first time

Mario Herrero, systems analyst at the Africa-based International Livestock Research Institute (ILRI), is co-author of a paper to be published today in the prestigious US Proceedings of the National Academy of Sciences (PNAS). The paper quantifies the role of livestock as a nutrient source globally for the first time. The paper, ‘A high-resolution assessment on global nitrogen flows in cropland’, reports results of an investigation of the sources of nitrogen for crop production globally. ‘We quantified the role of manure in different continents and in different agricultural production systems,’ says Herrero. ‘We found large differences in manure levels. In large parts of Africa and South Asia, which have the greatest numbers of poor people in the world, most of whom make a living by farming, manure can represent 35-40% of the nitrogen needed for growing crops, making it a major source of needed nutrients in these regions,’ Herrero. Elsewhere, he explained, where farmers have ready access to chemical fertilizers, manure plays a less important role in crop production. The paper shows that livestock manure is as important a nutrient contributor as (and in some regions, is even more important than) the stalks, leaves and other wastes of crops after harvesting, which are often fed back into soils to help enrich them for the next cropping season. But those crop residues are becoming increasingly scarce due to their competitive uses. And one of the biggest competitive uses is as animal feed. Many farmers are loath to put their crop residues back into their soils because they need them to feed their animals. In South Asia and sub-Saharan Africa, crop wastes represent between 40 and 60% of all the feed for the cattle, sheep, goats and other ruminant animals raised. ‘Crop residues are a hugely important resource,’ says Herrero. ‘And needing to keep these resources to feed their animals stops many farmers from adopting conservation agriculture, which requires putting the residues back into the ground.’ Of course, the animals consuming crop residues deposit their manure on the ground. This analysis by Hererro and colleagues suggests that, globally speaking, livestock manure and crop residues make similar levels of contributions to nutrient levels. ‘In developing countries,’ he says, ‘the best solution is often for a farmer to feed her crop residues to her ruminant animals and then fertilize her soils with the manure they produce.’ That’s because these farm animals provide poor farmers with many other essentials as well, including highly nourishing animal-source foods for the household, much-needed year-round cash incomes, and draught power, transport and other inputs for successful cropping. ‘The bad news,’ says Herrero, ‘is that the amount of manure we have in Africa and South Asia is not nearly enough to increase levels of crop production. And to feed the world’s growing human populations, we’re going to have to increase the amount of nutrients we’re providing the soils in these regions.’