Identification and management of grassy off types in Grain Sorghum fields

Transcription

1 Identification and management of grassy off types in Grain Sorghum fields Key points: Grassy sorghum off types/out cross s have a low potential to become problem crop weeds Control measures are the same as for all annual grass s and volunteer grain sorghum Grassy off types have significantly less weedy potential than Johnson grass Seed off these are not toxic and are not declared weed seeds The grassy offtype in the line of MR43 has been formally identified as Sorghum Bicolor NOT USA VERSION OF SHATTERCANE AS PERPORTED BY SOME PEOPLE IN THE SEED INDUSTRY Every year grassy off types can be found in all commercial grain sorghum fields, the large majority of these are parental types or what are commonly known as tall mutants. Expression of tall mutants is normally enhanced under good growing conditions (Figure 1) Tall Mutants are true to type but no longer express the same level of dwarfing as the rest of the hybrid. Figure 1: Irrigated grain sorghum with tall mutants Darling Downs QLD 2010 Source: Trevor Philp: Pacific Seeds Summer Grains Agronomist

2 The term grassy off type describes the crossing of grain sorghum and other members of sorghum genus, contamination of grain sorghum with hybrid forage seed can also occur. According to J. MC Donald; 1981 the three pollen sources for crosses with grain sorghum can be divided into three groups: 1. members of the sub-section Arundinacea (Sorghum verticilliflorum, S. Sudanense) 2. members of the Halepensia (S. Halepensia, S. almum) 3. hybrids resulting from crosses between the two groups. Members of the Arundinacea group include Sudan grass and Sorghum verticilliflorum, both cross readily with grain sorghum and are non-rhizomatous, while crosses of Sudan grass and grain sorghum are unsightly they pose a very low risk of becoming a weed as the resultant cross is generally sterile or have very low fertility (figure 2). Sorghum verticilliflorum is a tufted shattering perennial, out crosses to grain sorghum from this spp can result in shattering grain types; the level of fertility of this type are unknown; work is currently underway on a suspected cross of S. verticilliflorum and grain sorghum. Purseglove (1972) pointed out these types of crosses generally have very low fertility, thus pose a low risk of becoming a serious weed. Figure 2: Sudan out cross in grain sorghum Source: Trevor Philp: Pacific Seeds

3 Members of the Halepensia are easily distinguished from Arundinacea as they all have rhizomes; the most notorious member of this group is the serious weed Johnson grass (figure 4). Fertilisation from Johnson grass occurs at very low levels; Webel et al (1970) determined that out crossing between Johnson grass and grain sorghum in the presence of S. bicolor pollen would as low as.01%. Seed production of grain sorghum is confined to areas of no or very low populations of Johnson grass. Due to the very low frequency of out crossing and the hygiene practices of modern seed production the risk of weedy cross from Johnson grass is unlikely. Figure 4: Johnson grass head and roots; note rhizomes Source: Trevor Philp: Pacific Seeds In the past 4 seasons the appearance of a tall open headed shattering grassy off type has been observed in a number of locations and varieties; frequency of this type has been very low to date;.0004% of plants. Some fields this season have increased frequency of this out cross; to.001%. Plants of this type are 2 meters tall, Sudan grass in appearance, have swollen nodes, generally have 3 to 4 grain bearing heads. Inflorescence has been generally uniform; seed is red to brown with black glumes. Plants are similar in maturity, some a 3-5 days quicker, seed readily falls from the head at or just prior to harvest ( figures 5,6,7). Tests are under way to determine the fertility of this outcross, and early indications suggest that it is very low around 20%. Monitoring of fields previously identified as having this outcross suggest that survival of this type is low, fields that have been continuous no till sorghum for 4 seasons have showed no spread of this off type but as demonstrated some regression to the original parent material; which seems to be related to the Arundinacea group. Control of this out cross in sorghum fields would be the same as volunteer grain sorghum.

4 Figure: 5 Grain sorghum outcross: with both shattering and non-shattering heads Figure: 6 Grassy offtype with dark glumes Figure: 7 Seed of grain sorghum (left) and grassy off type (right)

5 Source: Trevor Philp & Andrew Short: Pacific Seeds In northern, south-eastern, south-central and western USA the annual grass shattercane, (a form of Sorghum bicolor) (Figure 8), can be difficult to distinguish from S. halepense in the field at some stages of development. It lacks rhizomes but has become a serious weed in grain sorghum and maize as it can be dormant and last for long periods in the soil. It spreads through seed and generally highly fertile. The seed can be black to brown in colour and larger than Johnson grass( figure 9). The head type can vary to being very open to very tight. To our knowledge there is no US shatter cane present in Australia, as stated above there are shattering types of wild sorghum in Australia. Figure 8: US Shattercane Figure 9: US Shattercane seed (left) and Johnson grass (right) Source: Missouri State University

6 Control of grassy offtypes The control of grassoff types in fields should be treated as you would any annual grass including volunteer grain sorghum, obviously control of summer grass s will be more difficult in crops of grain sorghum and corn as no incrop selective herbicides are available. Some cross s will set almost no seed and have low seed fertility, others can set up to 6000 seeds but have low levels of fertility. Over the past season we have evaluated the level of fertility in the Sorghum out cross observed in a line of Pacific MR43 in the season including a second generation of seed to give more accurate advice on the potential behaviour of the grassy offtype. The level of viable seed and frequency in the orginal crop will determine the weed management strategy. As with all summer annual grass s the best control measure for high populations in grass crops is rotation or fallow, cheap effective chemical and cultural control exists for these outcross (see Table 1 Weed Risk and Control of Grassy offtypes). Fields with low infestation do not have to be rotated out of sorghum as the seed of these offtypes is generally low in fertility and are suceptable to S-Metolachlor formulations. Montioring of fields that have had seed set from grassy offtypes from other hybrids outside of the Pacific Seeds range that have been in continuous sorghum rotation for 4 years have had low persistance (figure 10). Figure 10: Weedy sorghum offtypes in a field of continous grain sorghum production on the Darling Downs QLD

7 Pre-emergent control with S-Metolachlor is variable as it is with all annual grass s and other weed control measures will be need to completely remove these plants. Shield spraying, spot spraying and interrow cultivation will be effective against these outcrosses. Table one: Weed potential and control options for Grassy sorghum outcross, Weed Characteristics Comments Declared weed spp New weed Annual or perennial Annual Rhizomes Parasitic Viable seed Yes High levels >95% germination rate Dormancy Low Toxic: Grain Toxic Plant material Yes Prussic Acid, lower than grain sorghum Number of chemical control measures Yes Group A,B2,M Control Measures Mechanical: Fallow t recommended Avoid deep burial as may prolong viability; shallow working may encourage germination Mechanical: in-crop Yes Chemical: fallow Yes Group M Chemical: pre-emergent * reg: US data suggest control from Group K,D and BII Chemical: In crop Yes Group A,M Chemical: in crop Summer grass Rotational crops Yes Summer Yes Mungbean, Sunflower, RR Cotton Winter Yes All Grazing * Quarantine for a week to limit spread from undigested grains Machine Hygiene Yes Clean down headers Clean down spray coupes FARM HYGIENE: IT IS IMPORTANT TO ADVISE YOUR HEADER/COUPE DRIVER TO ENSURE THEY CLEAN DOWN ALL EQUIPMENT AFTER THEY LEAVE A PADDOCK CONTAINING THE MR 43 OFFTYPE TO ENSURE CONTAINMENT OF THE ISSUE. For further information please contact your local Pacific Seeds reseller or Maree Crawford, Sorghum Business Manager, Pacific Seeds. Phone: Mobile or Barry Croker Managing Director, Pacific Seeds head office