Gibberellic acid (GA3) is a natural hormone that promotes plant growth. When you use GA3 on pasture, it encourages growth by using the plant's energy, leading to more leaves and stems. This is especially useful during times when you expect less pasture growth. After applying GA3, you'll notice taller and slimmer leaves, sometimes a bit yellowed. To get the most out of GA3, you should apply it within 5 days after grazing and then graze again 3-4 weeks later. Be cautious: using GA3 can sometimes mean slower growth in the next grazing cycle. For best results, plan carefully and monitor the pasture's growth.
Gibberellic acid (GA3) is a naturally occurring hormone that is involved in regulating plant growth.
When applied to pasture, GA3 stimulates growth through mobilisation of plant energy reserves, resulting in leaf and stem elongation. The application of GA3 can be used as a tool to manipulate pasture growth and assist in matching feed supply and animal demand in the shoulders of the season when anticipated pasture growth is low.
Key points
When moisture and nitrogen are not limiting dry matter responses typically range from 200-500 kg DM/ha at about four weeks after application.
Apply in late winter/early spring, to pull feed forward ahead of balance date; or, in autumn, to increase feed supply
Apply within 5 days after grazing, when there is adequate green leaf present for uptake.
Graze treated pastures within 3-4 weeks after application
Be aware of the potential for reduced growth (lag) in future rounds and factor this into feed planning
What can I expect?
Longer, narrower and more upright leaves can be seen within several days of application and are often accompanied by a slight yellowing of pastures.
At recommended application rates, initial DM responses typically range from 200-500 kg DM/ha four weeks after application. Responses are greatest when soil moisture and nutrients are not limiting.
Responses to GA3 occur quickly, with visual signs occurring within several days of application. In contrast, responses to N generally take longer to become apparent and occur over a longer period. Therefore, the growth responses to GA3 and N are complimentary and result in a relatively consistent response throughout the regrowth period.
As GA3 affects the height of pasture, care should be taken when interpreting rising plate meter (RPM) measurements from GA3 treated pastures as there is the potential for overestimation of yield.
When should I use gibberellic acid?
GA3 is best used to fill anticipated feed deficit. Pasture DM responses to GA3 are strongly influenced by season with the greatest responses occurring in late-winter/early-spring and autumn. Apply within 5 days after grazing and graze 3 to 4 weeks after treatment to capture the maximum response.
Applying GA3 in late winter/early spring appears to replace the requirement for accumulated long days to initiate stem elongation. As the number of long days increases, plants begin to produce sufficient GA3 themselves. As a result, there is very little response to GA3 applied in late spring and summer.
GA3 is typically applied:
In late winter/early spring, to pull feed forward ahead of balance date; or,
In autumn, to increase feed supply and lactation length, provided this does not compromise average pasture cover and body condition targets at calving
To capture the maximum benefits of GA3, apply the following guidelines:
Apply within 5 days after grazing.
Responses decline considerably when applied beyond 5 days after grazing
Although there is no withholding period, GA3, is ineffective if applied before grazing as it is not absorbed into the un-grazed areas of the plant.
Graze treated pastures within 3-4 weeks after application
Limited trial data suggests that yield responses are maintained or slightly reduced through to 5-6 weeks with little significant yield responses if grazed beyond 8 weeks after application (Edmeades 2009; Van Rossum 2013).
GA3 may be applied as a tactical response to a projected feed deficit or used strategically to enable changes to a farm system (e.g. earlier calving).
GA3 needs to be applied in anticipation of filling a feed deficit and, therefore, requires careful planning, including assessing the amount of extra feed that GA3 will grow and when it will become available for grazing.
Due to the limited application window, GA3 cannot be applied to the entire farm at once. Rotation length dictates the area that GA3 can be applied to each week. As a result, GA3 application will not result in a rapid increase in total farm cover. However, it will increase feed supply over time. This differs to the use of N where a large portion of the farm can have N applied at once to increase pasture cover over the whole platform.
