Although it is common to alternate between till and no-till practices, past research has considered farmers’ tillage options to be limited to the dichotomous choice of whether or not to switch to a long-term no-till regime. This paper expands farmers’ options and models their choices of tillage frequency. Less frequent tilling sequesters more carbon but permits a greater accumulation of weeds, whereas more frequent tilling eliminates weeds but releases carbon (tillage emissions). The timing of tillage balances its marginal benefits and costs. Higher payments from industry or government for atmospheric greenhouse gas reductions will increase marginal cost and reduce tillage frequency. Other key parameters, such as higher rates of tillage emissions or reduced weed impact, also influence tillage frequency. However, for the discount rate and the natural decay rate of carbon, the net change depends on the magnitude of other parameters.