Chapter 8
Guidelines for Nitrogen Fertiliser Use

 

Frank McKenzie

DNRE, Warrnambool

 

 

 

When can nitrogen be applied?

 

·       Nitrogen fertiliser helps grow more grass.  Apply nitrogen when your herd’s feed demand is likely to exceed your pasture’s 'normal' production levels (e.g. winter).

·       Consider the season.  Spring applications usually give the best responses, followed by autumn and then winter.  Often, however, extra grass grown in winter is more valuable than extra grass grown in spring.

·       Apply nitrogen 2 to 8 weeks before you anticipate a feed shortage:

-     2 to 4 weeks for spring conditions;

-     3 to 5 weeks for autumn conditions; and

-        4 to 8 weeks for cold winter conditions.

 

Tip:

*          stagger applications to provide a continuing source of extra feed.  This will also cover a failure to respond in one paddock (say due to poor soil moisture or temperature) and also allow better control if climatic conditions improve to the extent that too much feed is produced all at once!

 

What state must the pasture be in before applying nitrogen?

 

·       Apply nitrogen to pastures that are actively growing.  This will help ensure that the pasture is better able to utilise fertiliser nitrogen.

 

Tip:

*          apply nitrogen to ‘green’ pasture with a height of 4 to 8 cm.  This is often a good indication of an actively growing pasture that is likely to respond well to nitrogen.

 

What kind of pasture will respond well to nitrogen?

 

·       Select a pasture that is likely to give a good response to nitrogen. Paddocks that are fertile and well drained respond well to nitrogen.

·       Select a pasture with a good proportion of ryegrass as this pasture species is likely to respond well.

 

 

Tip:

*          inspect pastures to see whether urine patches indicate that pasture will respond to nitrogen (urine patches should be darker green than surrounding areas and should display an obvious dry matter response).

 

Should nitrogen be applied before or after grazing?

 

·       Nitrogen is most effective when applied soon after grazing (e.g. 1200 to 1400 kg DM/ha or approximately 4 to 8 cm) and is less effective when applied to a pasture with more than two weeks regrowth.

·       Pasture nitrogen requirements are highest during the active growth that takes place in the two weeks after grazing.

·       Nitrogen may also be successfully applied a few days (< 5) before grazing.

 

How much nitrogen should be applied  at one time?

 

·       Apply nitrogen at a rate of up to 60 kgN/ha.

·       Lower rates (30 kgN/ha) often give better responses than higher rates.   Such rates would apply to autumn and winter.

·       Higher rates may be economically viable during spring when the pasture’s growth potential is at its highest.

 

How often can nitrogen be applied?

 

·       How much and how often nitrogen is applied depends on how much extra feed one wishes to produce.

·       Remember: if more nitrogen is applied at any one time than the pasture system can use, the extra nitrogen will be lost and this is costly.

·       The rates mentioned above (e.g. 30 to 50 kg N/ha) can be applied after every grazing or every second grazing, depending on how long the grazing rotation is.

·       Normally, in order to retain clover in the pasture, one would not apply more than 200 kg N/ha/year.

 

What nitrogen fertiliser type gives the best responses?

 

·       Under practical farming situations, different nitrogen fertilisers (e.g. urea versus ammonium nitrate) generally grow the same amount of grass.

·       Provided no essential elements (P, K and S) are lacking in the soil, use the cheapest source of nitrogen.

 

Tip:

*          remember that a fertiliser like urea is more susceptible to volatilisation than ammonium nitrate if applied under dry conditions.

 

What pasture growth responses can be expected from nitrogen fertiliser?

 

·       As a guide for dairy pastures in south west Victoria, one could expect to get:

-     10 to 15 kg DM/kgN applied in autumn;

-     5   to 10 kg DM/kgN applied in winter; and

-     15 to 25 kg DM/kgN applied in spring.

·       These expectations are based on the assumption that soil fertility is good, ryegrass content in the pasture is good and that soil moisture is not limiting growth.

·       Remember, anything that limits pasture growth (e.g. soil moisture, temperature, soil fertility etc.) will reduce the expected response to nitrogen.

 

Tip:

*          if you have gone to the expense of applying nitrogen fertiliser to grow more grass, make sure your pasture utilisation is good.  The more grass that is wasted, the greater the expense of the extra dry matter grown.

 

How long should one wait before letting cows graze nitrogen fertilised pasture?

 

·       To minimise the risk of nitrogen related disorders, grazing should not take place until 21 days after applying nitrogen.

·       This is not always practical and cows should be carefully monitored if grazing nitrogen fertilised pasture earlier than this.

 

Tips:

*          do not allow hungry, unadapted animals unrestricted access to nitrogen fertilised pasture; and

*          strip graze or allocate small areas initially.

 

Does nitrogen fertiliser use influence soil pH?

 

·       Over time the use of nitrogen fertilisers will acidify the soil and liming will be necessary.

·       It is interesting to note that retaining good levels of clover without ever using nitrogen fertiliser also acidifies soil.  This is because clover fixes nitrogen that has the same acidifying properties as fertiliser.

 

Tip:

*          soil pH can be monitored through regular soil testing.

 

Does nitrogen use affect the balance of clover in a pasture?

 

·       Nitrogen fertiliser does not directly knock out clovers.

·       Nitrogen fertiliser can make clovers lazy so that they fix less atmospheric nitrogen.

·       Nitrogen fertiliser can also make grasses a stronger competitor against clover so that clover gets shaded out.

 

Tips:

*          loss of clover can be minimised through careful grazing management (i.e. do not allow ryegrass to overtop and shade out clover); and

*          apply nitrogen during periods of low clover growth (May to September) to minimise competition between clover nitrogen fixation and fertiliser nitrogen.

 

How is nitrogen likely to be lost?

 

·       Nitrogen can be lost in dung and urine.  Nitrogen recycling is inefficient with up to 60 % of nitrogen in urine patches being volatilised or leached.  High losses of nitrogen can occur when cows spend a long time in laneways.

·       Nitrogen can be lost through volatilisation (loss of nitrogen in the form of ammonia gas).  Urea and urine patches loose nitrogen under dry, warm, humid and windy conditions.

·       Nitrogen can be lost by leaching (nitrogen in the nitrate form moves with soil water and drains out of the plant root zone).

·       Nitrogen can be lost by denitrification (loss of nitrogen when soil nitrate is converted to nitrous oxide, a greenhouse gas).  This form of loss occurs under warm waterlogged conditions.  Fortunately, soils are usually waterlogged and cold in winter.  Risk periods can be around the autumn break and in late spring on waterlogged soils above 10 oC.

 

Tips:

*          reduce the time period that cows spend standing on laneways;

*          avoid applying urea under warm dry conditions;

*          avoid applying nitrogen to waterlogged soils; and

*          lower rates (< 50 kg N/ha) of nitrogen reduce the risk of nitrogen loss.


Nitrogen Decision Support System Look-up Table for Western Victoria (compiled by Frank McKenzie)

Average N response (kgDM/kgN)

Pasture Index

Jan*

Feb*

Mar*

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov*

Dec*

Low

**

**

**

5

5

4

4

7

8

8

**

**

Medium

9

9

9

8

7

5

5

8

14

14

11

11

High

14

14

14

10

9

8

8

12

20

20

15

15

% Range in actual

22

22

22

35

32

34

34

27

25

25

24

24

Typical Response Time (days)

21-32

21-32

25-35

28-36

28-42

36-90

21-32

18-28

14-28

14-21

14-28

18-32

Notes

Ryegrass

Olsen P

Colwell K

pH (H2O)

Low =

< 30%

< 12 ppm

< 80 ppm

<4.5

High =

> 60 %

> 25 ppm

> 275 ppm

>5.5

Comments:
* = Assumes irrigation, as summer rainfall in SW Victoria (Nov - Mar) is often too unreliable to allow for N applications
** = would not irrigate low potential pasture
Responses need to be adjusted within the range given, based on rainfall and temperature for the current season.
Responses are quote for average responses to nitrogen applied early in the month and responses measure late in the same month
Summer nitrogen responses are highly dependent on adequate soil moisture, usually only adequate with irrigation
Short regrowth times (eg 14-16 days) may reduce responses

 


Nitrogen Decision Support System Look-up Table for Gippsland (compiled by Richard Eckard)

Average N response (kgDM/kgN)

Pasture Index

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

Low

7

3

4

7

5

5

6

8

9

9

8

7

Medium

8

4

6

10

9

7

10

13

16

13

12

9

High

9

5

8

14

12

10

15

18

22

18

16

12

Flood Irrigated ryegrass - Maffra

10

10

11

14

12

10

15

18

22

18

16

12

% Range in actual

22

22

22

35

32

34

34

27

25

25

24

24

Typical Response Time (days)

28-35

28-38

28-35

21-35

21-35

35-90

28-42

18-28

14-28

14-21

14-28

21-32

Flood Irrigated paspalum - Maffra

18

18

10

7

5

5

10

10

12

12

14

15

Typical Response Time (days)

17-21

17-21

21-28

28-35

28-35

35-90

60+

30-40

21-30

18-21

18-21

18-21

Notes

Ryegrass

Olsen P

Colwell K

pH (H2O)

 

Low =

< 30%

< 12 ppm

< 80 ppm

<4.5

High =

> 60 %

> 25 ppm

> 275 ppm

>5.5

Comments:
Responses need to be adjusted within the range given, based on rainfall and temperature for the current season.
Summer nitrogen responses are highly dependent on adequate soil moisture.
Responses are for average responses to nitrogen applied early in the month and responses measure late in the same month.
Short regrowth times (eg 14-16 days) may reduce responses.
Flood Irrigation Comments.
Good surface drainage reduces waterlogging after flood irrigation.
Good late autumn rains may increase response of ryegrass in late autumn (irrigations often stopped in late April)

 


Nitrogen Decision Support System Look-up Table for Northern Irrigation Region (compiled by Geoff Mundy)

Average N response (kgDM/kgN)

Pasture Index

Jan

Feb

Mar

Apr

May

Jun

Jul

Aug

Sep

Oct

Nov

Dec

Ryegrass Low

6

6

6

6

4

4

8

8

9

6

6

6

Ryegrass Medium

8

8

9

9

5

5

10

10

13

9

9

8

Ryegrass High

9

10

11

10

7

5

15

15

20

15

10

10

% Range in actual

22

22

22

35

32

34

34

27

25

25

24

24

 Response Time (days)

21-30

21-30

21-30

28-36

28-42

60-90

60+

30-40

21-30

18-24

21-28

21-28

Paspalum dominant

20

20

9

6

5

4

5

10

11

12

13

15

Response Time (days)

17-21

17-21

21-30

28-36

28-42

60-90

60+

30-40

21-30

18-24

17-21

17-21

Notes

Ryegrass

Clover

Olsen P

Colwell K & pH

Other notes

Ryegrass Low =

< 30%

>50%

< 12 ppm

K & pH not
usually an issue

Summer weeds eg sedge

Ryegrass High =

> 60 %

20-40%

> 25 ppm

 

Paspalum dominant =

< 30%

10-30%

 

 

Comments:
Good late autumn rains may increase response of ryegrass in late autumn (irrigations often stopped in late April)
Responses need to be adjusted within the range given, based on rainfall and temperature for the current season.
Responses are for average responses to nitrogen applied early in the month and responses measure late in the same month
Short regrowth times (eg 14-16 days) may reduce responses
Good surface drainage reduces waterlogging after flood irrigation
July response can be similar to August with 60+ days response time - reduce response by 50% with 30 days response time

 

 Data from a completed project funded by DPI and the Dairy Research & Development Corporation

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Last modified:29 January, 2002                         Please Note: Disclaimer             Authorised and maintained by: Richard Eckard
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