Driving winter pasture growth rates

If autumn results in less than favourable conditions across a lot of the state, many may be going into winter with less pasture in front than desired. So how can you increase the pasture growth rates across the winter period to improve how much feed we grow?

Grazing management

Did you know that you can double the growth rates of pasture through winter through the grazing management of the pastures?

A farm in Gippsland grazed two paddocks side by side in winter. One paddock was grazed for two days, the other had stock in for six days. Both paddocks were strip grazed but stock were not excluded from re-grazing the areas of the paddocks they had access to the day before.

When pasture growth rates were measured a month later the paddock grazed for two days had a growth rate of 18 kgDM/ha/day. The paddock that was grazed for six days had half the growth rate at 9 kgDM/ha/day.  The difference in leaf size of the regrowing pastures was also vastly different between the two paddocks. The paddock that had been grazed for six days contained plants that were less than half the size of the plants in the two day grazed paddock. The much smaller leaf size was impacting on the amount of sunlight plants could capture, influencing the amount of energy they could produce and therefore reducing the growth rate.

Are there ways you could minimise the effects of back-grazing in the system? Options include running temporary troughs off your main trough and moving the new strip, fencing it behind. Or, using electric fencing to create a laneway in the paddock back to the water source, or perhaps run the strip fences wagon wheeled off the water point.

Urea

Another alternative to improve winter feed growth rates is using urea. As a nitrogen source to use in winter, urea is less prone to losses (leaching and denitrification) during the cold wet months.  If pasture is growing slowly, we would expect a response rate to the urea of 5:1 – that is 5 kg dry matter (DM) of feed grown for every kilogram of nitrogen applied. So, if we applied 40 kg nitrogen/ha, we could expect an additional 200 kgDM/ha to be grown compared to what would have been grown without applying the urea.  If pasture growth conditions are moderate, we could expect a 10:1 response rate. A single application of nitrogen fertiliser is most efficient when applied at rates between 25-50 kgN/ha (54-109 kg urea/ha)

Remember to keep the stock off the paddock for 21 days post urea application, as nitrate toxicity could be a concern if grazed too early.

The feed grown through the use of urea is a cheap source of feed, when assessed in terms of cents/Megajoules (MJ) of Metabolisable (ME).  See table 1 for a comparison of urea cost and its impact on the cost of feed grown (calculations are based on a feed value of 10 MJME/kgDM grass grown).  Note – when doing your own comparisons, ensure you are using a delivered and spread cost for urea.

Table 1: Cost of feed grown (c/MJME) at different nitrogen response rates

Cost of urea per tonneNitrogen response rate 5:1Nitrogen response rate 7:1Nitrogen response rate 10:1
$550/T2.391.711.19
$600/T2.611.861.30
$650/T2.832.021.41
$700/T3.042.171.52
$750/T3.262.331.63

As an example – if urea cost $700/T (3.04 c/MJME), it is the equivalent of hay that is 9 MJME/kgM costing $219/T.  So, if you can source hay of that quality at that price and/or below, it is better to use the hay.  Otherwise if your hay is costing more than that, even at that urea price and low response rate, the urea grown pasture is a cheaper source of feed.  Don’t forget to do the calculations to see if the urea will grow enough extra feed to meet requirements.  You may still need to source some hay, but much less than if not using the urea.

Gibberellic acid

Gibberellic acid (a naturally occurring plant hormone) can be useful through the cold winter months, and generally the colder the day time temperatures the better the response.  During mild winters, the response to gibberellic acid can at times be negligible. The rapid plant growth that can occur through the use of gibberellic acid is often lighter in colour, but this doesn’t affect the quality of feed on offer.

Phalaris based pastures are highly responsive to gibberellic acid with recommended rates of application of 2.5 to 10 g of gibberellic acid/100L water. Pastures that are dominant in perennial ryegrass, annual ryegrass or cocksfoot, it is recommended to apply gibberellic acid at 10-20g/100L water.  Growth stimulation is usually seen within  seven days of application and ceases around three-four weeks after application.

Once the average daily air temperatures are above 15˚C, gibberellic acid will not increase pasture growth as the plants naturally have sufficient levels for growth in this temperature range.

Note: the responses to the use of urea and gibberellic acid are dependent on adequate moisture levels in the soil system.  If soils are not moist enough to support growth without their use it is recommended holding off on applying either until soil moisture levels improve.

Page last updated: 02 Jun 2021