A double cropping system is where two crops are grown in the same field over__________.

A double cropping system typically refers to the practice of growing two distinct crops in the same field within a single calendar year, effectively maximizing the use of land and resources. The correct timeframe for this practice is 12 months.

In a double cropping system, the first crop is usually planted in the spring and harvested in the summer or early fall, followed by the planting of a second crop that is harvested later in the year. This practice can enhance productivity and soil health but requires careful management of planting and harvesting schedules to ensure that both crops are successfully cultivated within the growing season.

The other timeframes mentioned, such as 6 months, 18 months, or 24 months, do not accurately represent the standard understanding of double cropping, as they either imply a shorter duration than is feasible for the growth cycle of a second crop or extend beyond a single growing year. Therefore, the choice of 12 months aligns perfectly with the definition and understanding of double cropping systems in agriculture.