Think of your soil as a bucket, autumn and winter rainfall fill up the bucket and once the bucket is full it will overflow. As temperatures warm up and pastures start to grow, the bucket starts to empty and is only topped up with spring rainfall. This rainfall sets farms up for spring and early summer growth, however it is inevitable that the bucket will empty most years. Maximising the amount of pasture grown from this bucket of water is important and there are multiple management practices that can help farmers maximise pasture growth during this spring period.
Evapotranspiration is the biggest user of available water and with that in mind, we need to maximise pasture growth of the plant before water is lost from the (soil) bucket. The aim is to maximise spring growth and can be manipulated with correct grazing management, fertiliser usage, plant hormones and weed control.
So, what are the key nutrients required when coming out of a growthy winter? Typically, after a wet winter, nitrogen and potassium are in short supply as they are mobile nutrients. If the bucket (soil) has been filled with water to the point where it is overflowing, sometimes nutrients like potassium (K) and nitrogen (N) can wash out – a process we call leaching. If your pastures are limited by N or K there will often be strong urine patches seen in the image below.
In those urine patches there is a lot more plant mass, however the plants have used the same amount of water as the non-urine patch. Showing why it’s important to use N and K wisely in late autumn/early spring. Applying urea and a spring dressing of K will help overcome this.
Sulphur (S) is another nutrient that is often less available at the end of winter. When soils drop below 8°C, microbes stop converting organic sulphur into plant available sulphur. Sulphur is extremely important for protein production and for plants to uptake nitrogen. It’s important to be using plant available sulphur in early spring to help drive pasture growth, talk to your agronomist for specific recommendations.
Plant hormones such as gibberellic acid (GA) can also be used to lift pasture covers. In One50 perennial ryegrass, GA application resulted in a 465kgDM/ha yield increase over a 4-week period (Van Rossum, Bryant & Edwards, 2013). When applied with nitrogen the yield increase was 666kgDM/ha, showing the importance of applying them together (Van Rossum, Bryant & Edwards, 2013). This trial showed a similar result when GA was applied to tall fescue, with a 583kgDM/ha increase when applied with N. It’s important to note these results are only possible in a rotational grazing system where the GA + N are applied 1-5 days after animals are removed from grazing. So if using GA you need to ensure correct grazing management.
Grazing management is often the largest cause of wasted feed. A ryegrass plant needs to be grazed at the 3 leaf stage to maximise yield. If grazed before the third leaf is fully emerged, yield is limited as the third leaf is responsible for approximately 50% of total dry matter. This will often be when covers are at 2500-3200kgDM/ha. Once the 4th leaf starts emerging, this can lead to increased competition for nutrients (light), therefore increasing the risk of leaf senescence (death) or existing leaves on the plant. So net pasture accumulation is even and herbage quality starts to decline. It is also important to monitor the emergence of seed head. Once a grass starts to go reproductive, its quality drops significantly. Practical implications of this mean you need to speed up your grazing rotation, get more mouths on the farm or drop a paddock or two out for silage or hay production.
In summary if spring moisture is limited, you need to maximise the amount of forage grown from that water. Make sure your grazing management harvests as much feed as possible, make sure your pastures are well fed with the right nutrients and if practical, look at using gibberellic acid to help give you that much needed kickstart into the spring.
Van Rossum, M. H., Bryant, R. H., & Edwards, G. R. 2013, Response of simple grass-white clover and multi species pastures to gibberellic acid or nitrogen fertiliser in autumn. Proceedings of the New Zealand Grasslands Association 75: pages 145-150.