Selectivity is defined as the ratio between weed control and crop injury and pre-emergence tillage selectivity is high compared to post-emergence operations. As most weed seeds emerge from the top 2 cm of soil, significant weed control can be achieved by using tillage between seeding and crop emergence, especially with large seeded crops (cereals and pulses) that have been seeded deep. Research has shown that the rod-weeder is very effective at controlling weeds in field peas when used for pre-emergence weed control. Field peas and lentils have underground nodes that allow re-growth if tillage damages the hypocotyl and the cereal crop’s mesocotyl remains relatively close to the seed (compared to wild oats), which helps protect the growing point from injury. Evaluate crop germination carefully and choose a day with sunny, dry conditions to have the best results with pre-emergence tillage operations.
Post-emergence weed control operations have lower selectivity and it is important to consider the effect that different tillage implements will have at varying crop and weed stages. The most commonly used in-crop tillage implement in organic production is the harrow (tine and flex-tine) and recent research on the rotary hoe has demonstrated its effectiveness in organic production systems.
Harrow type (tine, rotary or flex-tine) has minimal effect on selectivity; however, crop and weed stage, tine angle setting, implement speed and post-harrowing weather conditions all impact crop recovery and final yield significantly. Aggressive tine angle settings increase crop burial and decrease selectivity, thus to reduce crop injury while maintaining weed control it is recommended to set the tine angle around 45°. In past years it has not been recommended to use tine harrows in oat; however, recent research has shown that oat is as tolerant to tine harrowing as wheat or barley. Refer to Factors affecting in-crop harrowing crop recovery & Crop staging recommendations for post-emergence harrowing for factors that will improve harrowing success and the recommended crop stages for in-crop harrowing, respectively.