A dry winter/spring, like many areas of NSW experienced in 2017, is why it is desirable for annual legumes like sub clover and serradella to be early maturing hard seeded varieties.
For us and for many other farmers, 2017 experienced good germination conditions in March/April when annual legume components of native and introduced pastures germinated. Unfortunately follow-up winter and spring rains were commonly very low to low, a not untypical situation, although 2017 would rate as particularly severe as far as rainfall was concerned.
Early maturing varieties of many introduced and naturalised clovers (like burr medic) began flowering from late July early August onwards for many areas.
Except in total drought conditions, where plants died before late July, commonly varieties flowering this early can set at least replacement seed for next year’s autumn regeneration. Even for so called reliable late winter spring rainfall environments, seasons can be erratic and sometimes dry late winter/springs can occur in consecutive or even more frequent years. Mid to late maturing varieties fail to set much if any seed for future years in these situations.
Older introduced annual legume varieties (typical for up until the 1960s) were commonly mainly “soft” seeded types. Soft seeds are vulnerable to germinating “off season”, which for winter legumes means in summer as a result of storms or other significant rain events.
For most areas germination “off season” results in relatively early seedling death as conditions are too hot for these plant types. Soft seeds germinating from false breaks quickly results in total loss of soil seed reserves.
Pasture plant breeders from the 1960s onwards sought out winter legume species and varieties with both early maturity and high levels of hard seed to ensure long term persistence, even in drier environments, but also for medium to better rainfall areas.
“Hard” seed is defined as protected from germinating “off season”, in the case of winter legumes from summer early autumn rain events. Various types of “hard” seed mechanisms exist in specific species and varieties and typically mean seeds are impermeable to water and will not germinate until the seed coat becomes permeable. Often permeability occurs over summer, in time for the mid to late autumn break, as a consequence of hot day and cool nights that breaks down or degrades the seed coat.
Many varieties of some species have their “hard” seed break down gradually over several years, which means some viable soil seed reserves can live dormant for several years.
These species and varieties are capable of regenerating even if several consecutive dry late winter and springs occur. Also these can often survive a few years of cropping where legumes would be destroyed by various herbicides used to control in-crop weeds.
Sub clover (most varieties) after flowering buries a lot of its seed, a good drought avoidance feature when combined with early maturity and “hard” seed. While overgrazing can reduce seed set subs burying ability helps ensure some seed set even in difficult conditions.
Many of the newer species like serradella, biserrula, gland, arrowleaf and bladder clover are aerial seeders. So are many older species like barrel medic and rose clover.
Aerial seeders need to be periodically carefully grazed, especially in the initial seed soil build up phase after establishment, and after droughts, to ensure a return to a solid soil seed reserve.
Soil fertility and ensuring adequate levels of rhizobia bacteria are other aspects of persistent productive pasture legumes.
Next week: Research examines hybrid verses open pollinated canola varieties.
- Bob Freebairn is an agricultural consultant based at Coonabarabran. Email firstname.lastname@example.org or contact 0428 752 149.