15 IREC Farmers' Newsletter No. 199 — Autumn 2018 increased and cotton is one of the few crops that can be forward sold up to three to four years out. Even the 2016–17 season, which was a difficult growing season, gave a positive result for most growers. Based on 15 responses to an annual survey, data collected showed that the 2015–16 season was exceptional for yield at 12 bales per hectare while 2016–17 was a difficult season, weather wise, with farm yields between 7 and 11 bales/ha. Data collected about returns for the past two seasons is summarised in Table 2. Note these are average figures with GM/ha varying from $300/ha to $3,500/ha. What’s the problem with ‘back to back’? One of the major threats to ongoing good returns for cotton is the temptation to increase the area of back to back cotton. Growers regularly ask me, “How long can I go back to back?” Ideally it would be good to see no back to back cotton but in years with good water availability, the area of back to back cotton will increase. However, this is a ticking time bomb in terms of disease build-up. Prevention and good management practices are the only measures available at present to combat disease. Cotton growers need to have realistic expectations of yields from back to back cotton. The yield penalty can be at least 1.5 bales/ha compared with fallow. I have seen farms with long-term back to back where the yield decline gets to a point where losses occur. We have a different growing environment in the south compared with the northern cotton regions and we need to make realistic management decisions on long- term rotations. Disease build-up The southern environment can be a challenge at the start and the end of the cotton growing season. Cold starting temperatures can slow cotton seedling development and it makes it difficult for seedlings to ‘outrun’ disease. Black root rot and the interaction with other seedling diseases can stall a crop’s development. Black root rot will not kill seedlings but it steals crop development time in the short southern season environment. Black root rot spores will build up over time when a host is grown in the field. Cotton is the main host and we are now seeing patches of black root rot in Murrumbidgee fields after about five or six cotton crops — even with rotation with cereals. However, a sensible cotton– Table 1. Cotton growing figures for southern NSW and northern Victoria, 2017–18 Location Crop area (ha) % increase on last season Total no. of growers No. of new growers Lachlan 18,388 53 39 9 Murrumbidgee 67,587 56 164 53 Murray & Victoria 4440 43 13 (NSW) 3 (Vic) 5 Table 2. Gross margin returns for cotton in the southern growing region Season Income/ha Variable costs/ha Gross margin/ha Gross margin/ML 2015–16 $7200 $3895 $3305 $330 2016–17 $5376 $3395 $1971 $209 Black root rot has been observed in the Murrumbidgee region after five or six cotton crops — even with rotations with cereals. Other crops also host black root rot so careful rotation planning is required. wheat–fallow rotation will delay disease build-up and help reset the compaction in the field. Legumes also host the black root rot fungus, so crop choice should be carefully considered when setting up rotation plans. Hillston growers with a longer history of cotton now have problem fields with black root rot. In the warmer Hillston environment, crops may outgrow the disease but top end yields are not reached in these patches. One grower estimates the difference can be 14 bales/ha in the good areas and 8 bales/ha in the affected areas. A recent meeting with industry pathologists at Hillston indicated that there is no one solution to the problem other than fields going out of cotton for three years. The cotton breeders are working on a genetic solution but that is still a number of years away. The 2017–18 season also saw early infections of alternaria leaf spot in back to back cotton. This caused setbacks to some crops and it may be a more common event in our environment following a dry winter coupled with a cool start. Turning fields around in time, over winter, for the next cotton crop is full of challenges and compromises. It can result in fields with poor seedbeds. Figure 1. Cotton growing area in the southern region, 2004–18 Area (ha) Southern NSW cotton area