16 IREC Farmers' Newsletter No. 199 — Autumn 2018 Rotation Gross income ($/ha) Total variable costs ($/ha) 10-year gross margin ($/ha) Gross margin ($/ML) Back to back 36,180 27,767 8413 115 Cotton–fallow 30,000 18,625 11,375 227 Cotton– wheat–fallow 35,870 23,375 12,495 192 Table 3. Returns of three cotton rotations Compaction The end of the cotton growing season normally sees temperature fall, evaporation rate drop and the start of the autumn break. This can lead to a wet pick with increased compaction resulting in soil structure decline. If this happens, fields should be left out of back to back cotton, and instead, a cereal crop grown to repair compaction at depth. A recent project run by the Southern Valleys Cotton Growers Association, funded by Cotton Research & Development Corporation (CRDC), looked at measuring soil compaction on irrigated farms that have been exposed to heavy machinery, as well as multiple cultivations, and compared the structure with that of a controlled traffic farm. There was also monitoring of water infiltration over a cotton season to assess if there were negative effects being seen due to compaction. Data was collected from three cotton fields, each with slightly different management histories, as well as from a dryland farm that runs a controlled traffic system. The key findings l  Under controlled traffic, compaction can be limited to the top 10–20 cm of the soil profile. l  Depth of compaction in irrigated soils is quite deep and difficult to fix. A dry pick would require a 40 cm deep rip, a wet pick would require a 60 cm deep rip. l  Beds suffer much worse compaction than hills. Furrows seem to crack more and therefore somewhat fix themselves. l  Old-school thinking is that compaction is in the furrows. The reality is that in irrigated systems the big machinery on beds causes much worse compaction then seen in furrows. l  Slightly different A–B lines each year creates more compaction across a wider area — use set A–B lines. l  Water infiltration is less in the compacted zone compared to guess rows — meaning that the ‘bucket’ available to the plant is reduced. Rotations in the long term Not every year will there be enough available and affordable water for the area of cotton that growers want to grow. This is a major factor in thinking long term about rotations. If winter crop prospects look good, more effort and resources may go to maximising winter crop yields but with current winter crop prices, the majority of the farm income (could be 90%) will be from summer cropping. Early infections of alternaria leaf spot in back to back cotton occurred in the 2017–18 season, setting back some crops. The disease may be more common following a dry winter coupled with a cool start. Compaction can be monitored in a cropping field. Thane Pringle from Independent Precision Ag uses a P4000, pictured, to measure resistance in the soil profile. Data collected is converted to a virtual soil map that identifies changes in soil density to a depth of 90 cm. I am seeing a trend where cereals are grown after cotton as a low input crop that may or may not get a spring irrigation. The crop is primarily grown as a repair crop. There is also a trend of having more on-farm storages for irrigation water, giving the flexibility to hold off-allocation water that may be used to finish winter crops. When planning rotations, it is best to think of crop choices and the long-term implications of those choices. An interesting exercise is to do budgets out for 10 years and look at the overall returns and the sustainability of consistent returns. A rotation planner decision support tool was used to compare three rotations: l  back to back cotton l cotton–fallow l cotton–wheat–fallow. The back to back rotation had the highest gross income but also the lowest gross margin over a ten-year period (Table 3). Four crops were grown back to back before it had to be fallowed due to yield reductions. The back to back rotation had seven cotton crops in the ten years. The other rotations grew five cotton crops in the ten years. The cotton– fallow rotation used 50 ML in the 10 years with the best gross margin/ ML return of $227.