What is not useful to increase agricultural production?

An agricultural production-enhancing practice is an agronomic input -- an intervention in labour efficiency, nutrient supply, or water supply -- applied on land already under cultivation to raise the output obtainable from it. Clearing forest to bring new land under the plough is a land-use conversion, not an agronomic input: it might appear to add farmland in the short term, but that gain is outweighed by the resulting loss of soil fertility, accelerated erosion, and disruption of the local rainfall cycle that farming depends on, so it is officially treated as environmentally harmful rather than production-enhancing.

Checking each option against this principle:
- Mechanisation -- using machines such as tractors and harvesters -- is a genuine agronomic input: it speeds up ploughing, sowing, and harvesting, reduces losses from delayed operations, and lets a larger area be farmed with the same labour.
- Increased fertilizer use replenishes the nitrogen, phosphorus, and potassium that crops draw from the soil each season, directly sustaining or raising per-hectare yield -- a genuine agronomic input.
- Enhanced irrigation assures water supply through dry spells and enables multiple cropping cycles a year -- a genuine agronomic input.
- Deforestation is not an agronomic input at all -- it merely converts forest to farmland. The exposed topsoil erodes without root cover, nutrients wash away, and the disrupted local rainfall pattern reduces yields on that land and on neighbouring farmland, so the net effect works against agricultural production rather than for it.

Among the four listed measures, only Deforestation is not counted as a genuine method to increase agricultural production: unlike the other three, it is not an agricultural input at all, and whatever land it adds is offset by the resulting degradation of soil and water resources.