21 IREC Farmers' Newsletter No. 199 — Autumn 2018 Cotton grown after a summer fallow yielded 1.28 bales/ha or 13.8% more than cotton grown after a crop (predominantly cotton) the previous summer (Figure 3). Some locations were better able to manage back to back cotton and reduce the yield differential between back to back and fallow. Whether this difference between location and rotation is because of management or other factors is not clear from the data collected and warrants further investigation. Bed geometry The comparison of growing cotton on wide beds as opposed to one metre spaced hills has long been a point of discussion around the cotton industry. The results from the 2016–17 season show no significant difference between bed and hill configurations under fallow conditions. However there was a significant difference of 0.6 bales/ha (7.5% yield increase) in favour of hills compared with beds when the crop was grown back to back (Figure 4). When all rotations were grouped together the yield increase of hills over beds was 0.8 bales/ha or 8% in favour of the one metre hill. What is not clear from the data is the predominant type of irrigation system within each row configuration. The other variable may be that of compaction, with wet picking and the dual wheel of the picker causing more compaction on the top of the bed in the back to back system compared with the fallow system. In turn, compaction may cause reduce water infiltration and fertiliser recovery, and consequently lower yield. These issues and the management of back to back cotton field also need further research. Variety The predominant variety sown in the region in 2016, as indicated in the data, was Sicot 74-6B3F on 64% of the area. This was followed by Sicot 71-4B3F on 21%. The rest of the area was made up of Sicot 7 4-8B3F and Sicot 707B3F. As the 2016–17 season saw a change from the Bollgard®2 (BG2) technology to the Bollgard®3 (BG3) technology, similar breeding lines of Bollgard2 were grouped with Bollgard3 lines in the analysis. The survey results showed that the most popular variety (74-6) is the highest yielding variety with 0.72 bales/ha (8%) higher yield than the next popular variety 71-4 (Figure 5). The data also showed some differences between varieties between locations, however due to the small sample size of some varieties (one or two fields per location) little should be concluded from these mixed results. Over time, the database will determine optimal nitrogen rate, sowing date, variety and bed geometry for southern cotton production. Hectares Figure 4. The effect of bed geometry, in back to back cotton, on yield in the southern cotton growing region, 2016–17 Yield (bales/ha) Bed geometry in back to back cotton SE = 0.24 Bed Hill Average of yield Sum of ha Hectares (ha) Figure 3. Average yield of back to back summer crops compared with summer fallow in the southern cotton growing region, 2016–17 Yield (bales/ha) Rotation SE = 0.15 B/B Fallow Average of yield Sum of ha Average of yield Sum of ha Figure 5. Average cotton yield and area grown of different varieties in the southern cotton growing region, 2016–17 season. Note: Columns with different letters are significantly different at the 5% lsd level. Hectares (ha) Yield (bales/ha) Variety