Summer is a critical time in pasture production systems due to the limited amount of rainfall. Grasses have a bunch root system with most of their roots growing close to the soil surface. This area dries out in the summer limiting the growth of the grasses. Species with a tap root or bulb can extract water lower down the soil profile. Forage brassicas are a reliable feed source over the summer period when grasses are not growing. The most common forage brassicas are forage rapes (and their hybrids) and forage turnips. The differences between the two groups are that forage rapes can be grazed multiple times whereas turnips can be grazed hard only once.
With the onset of climate change Australia is experiencing much drier autumn periods. Forage brassicas can be used to create a feed bank for this period if a late autumn break means that grasses will not generate any feed until winter. Forage rapes are ideal for this situation. You can have the benefit of a summer grazing and then closing them up for an autumn grazing.
The two examples of trials below show the different ways in which forage brassicas can be used. The first trial is in the south west of Victoria, an area that traditionally uses forage brassicas as a source of feed over the summer. This trial was harvested after 12 weeks growth and then grazed with stock. Turnips give the highest yield at the first grazing due to the DM produced by the bulb. To get the full advantage of turnips you must get the stock to eat as much of the bulb as possible. By doing this the turnip will have no regrowth.
Figure 1: Forage brassica trial at Yambuk (South West Victoria) 2008/09 season (Sown 24/10/08)
Hobson rape is a pure forage rape with fast growth to first grazing between weeks 10-12. Other hybrid forage types such as Winfred and Pasja claim to have a quicker first grazing than straight forage rape types but this data shows this not to be the case. Late rainfalls in November 08 helped Hobson achieve high DM yields in the first harvest in January 09. The summer was then hot and dry with limited opportunities for growth. This was made up for in autumn DM production by the forage brassicas. Hobson produced another 5 tonnes of DM in autumn, giving a total of 13 tonnes of DM produce overall. Over this period of 7 months under trying conditions Hobson was able to produce as much DM as what a perennial ryegrass would in this area annual.
Figure 2: Forage brassica trial at Yarck (North East Victoria) 2008/09 season (29/10/08) with summer grazing (in blue) or autumn grazing (in red).
In comparison north east Victoria is above the great divide and experiences more hotter and drier summers than those experienced in south west Victoria. This is an environment that would put any forage brassica to the test. DM yields were half of those from the south west, but trends were familiar to those of the south west. Forage turnips had the highest yield of the first measurement. This trial was not grazed with stock to determine how much forage could be produced for autumn. Hobson was able to achieve 9366kgDM/ha over that 7 month period. This would be equivalent to the annual production of a perennial ryegrass in this area.
Forage brassicas such as Hobson are insurance for autumn feed if there is a late autumn break. Farmers wanting to sow them should follow these steps:
- Select a paddock that needs renovation and spray out with a knockdown herbicide in early August,
- General practice is to cultivate and then sow due to the small size of the seed but it is possible to direct drill after spraying if soil moisture is limited (rape=3-5kg/ha; turnip=1-2kg/ha),
- If broadleaf weeds are not a problem in the paddock think about sowing with a perennial clover such as white or red clover, and then grasses can be oversown into these in the next autumn,
- Keep an eye on insect pests such as Red legged earth mite and Diamond back moth, spray if present or graze,
- Slowly allow stock to adjust to brassicas, have hay nearby for ad lib feeding,
- Strip feeding allows the best utilisation.
For farmers that are sowing their forage brassicas late (eg December) or sowing after a summer storm due to lack of feed it may be advisable to sow with some shiroe millet. Shiroe millet is a tropical forage grass that can survive with minimal amount of rain and can be grazed at any height as it doesn’t have the prussic acid problem that sorghum has. However it can only be sown in soil temperatures of >12°C and will die with the onset of the first frosts.
Forage brassicas such as Hobson are an important tool in getting through dry summers and autumns and every farmer should consider sowing some when they are doing their annual pasture plan.