Where and how much N to apply to sheep pastures

 Malcolm McCaskill, Agriculture Victoria, Hamilton

 The Grasslands Productivity Program revealed a need to develop well planned drought strategies to reduce the severity of abnormal seasons (de Fegeley 1997).  Most drought strategies rely on reducing the demand for pasture by reducing stock numbers or by supplementary feeding.  However, nitrogen application can increase the amount of feed grown per unit of rainfall, and complement other strategies of coping with poor seasons.

Sheep.gif (3820 bytes)

 

Figure 1 shows simulated pasture growth rates at Hamilton in good and poor years.  Good years start with an early autumn break and have an extended period of rapid growth during spring.  Poor years start with a late autumn break, have a peak of spring growth in October as the pasture uses water stored in the soil from winter, followed by a sharp reduction in growth rates and an early drying off of the pasture.

 

 

 

 

Figure 1.  Simulated pasture growth rates at Hamilton for good and poor years, indicating when N could be used to boost pasture production.

 

Situations where a sheep producer could benefit from strategic application of nitrogen fertiliser are

·          After a late autumn break, N could be applied to develop a feed wedge more rapidly

·          If spring growth is likely to be below average, strategic application of N in August could ensure sufficient feed to maintain ewes and lambs until weaning in November

·          To increase yields of hay or silage crops

Pastures give their best N response when rapidly growing.  On fertile pastures with suitable species, N application can nearly double winter growth rates, while spring growth rates can be increased by 30%.  For these reasons, N application as a strategy for below average seasons is only an option for the period May to August.

Spring pasture growth is dependent on soil moisture stored during winter, and rainfall received during spring.  After a normal winter, there may be a store of 150 mm of plant-available water in the soil, compared with 180 mm of rainfall received during spring. 

Spring rainfall is reduced in El Niño droughts.  The effect is greater north of the divide than in the south.  Over the period of historical rainfall records, spring rainfall in El Niño years has been 29% below average at Balmoral, 22% below at Hamilton, and 17% below average at Heywood.

A combination of soil moisture status at the end of winter and the Southern Oscillation Index (SOI) can be used to indicate conditions under which it could be useful to apply nitrogen to boost growth rates during spring (Table 1).  El Niño droughts occur when the SOI is strongly negative.

 Table 1. How soil moisture at the end of winter, and the Southern Oscillation Index (SOI) in May-June affect subsequent spring growth at Hamilton (based on Austen and Clark 1999).  

Soil moisture end of winter SOI Spring growth Year
Low strongly negative (< -8) poor 1997  N ?
High strongly negative (< -8) average 1990
Low neutral (-8 to +8) unpredictable 1999, 1967
Low strongly positive (> +8) average 1998

 

It should be noted that it requires a combination of both low soil moisture at the end of winter, and a strongly negative SOI before a below average spring can be predicted reliably.  These are the conditions when N application could be considered to boost spring growth rates.  When the soil profile is full at the end of winter, an average to above average spring is likely even if the SOI is strongly negative.

Apply N to the best paddocks : The best responses to N occur when P fertility is high and there is a high proportion of an N-responsive improved grass species.  Annual ryegrass is highly responsive, followed by short rotation ryegrass then perennial ryegrass.  Studies on the Long-term Phosphorus Experiment at Hamilton in 1997 showed that on a perennial ryegrass-subclover pasture at an annual P application of 22 kg P/ha, each kilogram of N produced an extra 7 kg of additional dry matter (Cayley et al. 1998).  An N efficiency of 7 kg DM/kg N is barely sufficient to compete with grain.  However, at a P rate of 33 kg P/ha, each kilogram of N produced 10-13 kg of additional dry matter.

N rates : Rates of 30-50 kg N/ha per application are normally recommended in dairying.  Higher N rates may be subject to higher leaching losses and lower N efficiencies. In sheep pastures Cayley et al. (1998) have shown linear responses to at least 100 kg N/ha.  This means that if the area of highly N-responsive pasture is limited, it is better to apply N at 100 kg N/ha to the best 50 ha rather than half the rate to double the area.  When higher rates of N are used, it is especially important to maintain sufficient stocking pressure to avoid growth reductions because of internal shading.

N responses last 2 months : Unlike P, K and other nutrients, which have good residual effects, N responses typically only last for 2 months after application.  This means that to maximise the spring flush in a poor season, N should be applied by mid to late August.

Phalaris won’t reliably respond to additional N : A grazing experiment on a phalaris-subclover pasture at Vasey (40 km north of Hamilton) showed little response to fertiliser N applied in June and August in 1998 and 1999.  The only response was in 1998, when the N produced increased growth rates in November.  Fertiliser N applied shortly after the autumn break can also exacerbate phalaris toxicity.  For these reasons, N fertiliser cannot be recommended for at this stage for phalaris pastures.

Feed budgeting is necessary : The expense of N can only be justified if the additional feed is needed.   Typical El Niños in Western Victoria cause only a 20% reduction in spring pasture growth.  If a property is 20% understocked, there is no need for the additional feed.

Utilise dairy industry experience : Extensive guidelines have been prepared for use of N in dairying, and are available on the Web at www.nitrogen.landfood.unimelb.edu.au

References

Austen, E, and Clark, S (1999) The effect of climate zone variability on the perennial pasture zone of southeastern Australia. MODSIM 99 Proceedings, University of Waikato, New Zealand (Ed. L Oxley, F Scrimgwour and A Jakeman)

Cayley, J, McCaskill, M, Montgomery, J, Lewis, B, Sholz, T (1998) Pasture response to nitrogen in late winter depends on phosphorus. Proceedings Grasslands Society of Victoria Conference, Bendigo, June 1998.

de Fegeley, C. (1997) Grassland’s Productivity Program 1993-1997. Proceedings Grasslands Society of Victoria Conference, Hamilton, June 1997.

Data from a completed project funded by DPI

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