H. Kirchmann and M.H. Ryan, Nutrient Exclusivity in Organic Farming - Does It Offer Advantages? (PDF), su ipni.net. URL consultato il 28 dicembre 2016 (archiviato dall'url originale il 12 marzo 2016).
«A number of long-term field trials in Europe reveal that crop yields are on average 20% lower in organic systems that combine crops with animals and 33 to 45% lower in organic systems with crops alone, compared to their conventional counterparts»