review of nitrogen and stocking rate experiments for milk production in ireland

Review of Nitrogen and Stocking Rate Experiments for Milk Production in IrelandAuthor(s): T. F. Gately, W. F. O'Keeffe and J. ConnollySource: Irish Journal of Agricultural Research, Vol. 23, No. 1 (1984), pp. 11-26Published by: TEAGASC-Agriculture and Food Development AuthorityStable URL: http://www.jstor.org/stable/25556071 .

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Ir. J. agric. Res. 23: 11-26, 1984

Review of Nitrogen and Stocking Rate Experiments for Milk Production in Ireland

T. F. Gately, and W. F. O'Keeffe An Foras Taluntais, Johnstown Castle Research Centre, Wexford

J. Connolly An Foras Taluntais, Statistics Department, 19 Sandy mount Avenue, Dublin 4

Abstract

This paper reviews the milk production results obtained from six large-scale grazing

experiments in the Republic of Ireland. In these experiments, levels of nitrogen varying

from 51 to 495 kg N ha"1 and stocking rates varying from 1.54 to 3.90 cows ha"1 on a

whole-farm basis were evaluated on four freely drained and two imperfectly drained soils.

The effects of the nitrogen treatments were measured as the percentage change in

stock-carrying capacity at a given level of output animal"1. Twenty-four data points were

included in a model which had the form

Y= 100 [K/ (1 + A exp (B)) ?

1]

where A was a constant and B was a function ofthe variables N, site and time relative to the

start ofthe experiment. Using this model, three curves were derived, for the freely and

imperfectly drained sites, which showed the predicted change to a range ofN levels for the

first, second and third or later years.

The predicted rate of N which gave the maximum percentage change in carrying

capacity was about 300 kg N ha"1 on the freely and imperfectly drained sites. On the freely drained sites, the predicted response to N, at 300 kg ha1 was 28, 77 and 89% change in

carrying capacity in the first, second and third or later years. The corresponding predicted

increase on the imperfectly drained sites was 16, 44 and 52%, respectively. The small

increase in percentage change in carrying capacity in the first relative to later experimental

years was probably due to the presence of more clover in the first year of these experiments.

Introduction

Many experiments have been carried out in

Ireland, especially since the establishment

of an Foras Taluntais in 1958, to determine

the optimum rate of nitrogen to apply to

pastures for grazing cows. Since these

relatively large-scale nitrogen experiments

under grazing tie up research resources in

terms of staff, land, animals and equipment,

it was considered that the time was oppor

tune to review the results to date and to

examine the need for future work in this

field. This paper examines the milk production

11



12 IRISH JOURNAL OF AGRICULTURAL RESEARCH, VOL. 23, NO. 1, 1984

results obtained using dairy cows where

several levels of nitrogen and of stocking

rates were tested. Our findings are also

related to results from similar experiments

carried out in other countries.

Nitrogen use in Ireland

There has been a dramatic increase in the

use of fertiliser N in Ireland since 1945 (Fig. 1). The 275,000 t applied in 1980-81

corresponds to an overall application of

about 80 kg N ha"1 to the 3.4 m hectares of

good agricultural land. The most recent

fertiliser use survey (1) has shown that the

average use of N on mainly dairy farms is

about 125 kg ha"J.

In practice, there is a wide variation in

the use of fertiliser N on dairy farms with

275)- j

H

225- I

200- r

/ ? H / E 125- V

I / S. 100- / z i

75- /

50- /

1945 50 55 60 65 70 75 80 Year

Fig. 1: Consumption of fertiliser nitrogen in Ireland from 1945 to 1982

some farmers using N for silage only, others

using it for early grass and for silage and the

remainder using it on all the farm. The

actual rates vary from about 70 to 400 kg N ha"'.

Response to nitrogen

a) Grazing versus cutting: The optimum level of N to apply for dairy cow grazing is

determined not so much by the herbage

grown as by the proportion of the extra

herbage converted into milk, relative to the

cost of the N. In addition, the total

overhead and labour costs of keeping extra

cows as a result of N application should be

allowed for in determining the most

economic N levels. Evidence on the response

to N under large-scale grazing experiments,

either in terms of increased dry matter

production or animal product, is scanty.

Jackson and Williams (2) in six experiments over 4 years, with steers as the grazing

animal, got responses of 4.3 and 14.5 kg

DM kg !

N from grazing and cutting,

respectively, at 200-400 kg N ha"1. Laissus

and Jeannin (3) obtained responses of 5.3

and 9.7 kg DM kg"1 N from grazing and

cutting, respectively, with 162-320 kg N ha"1. Van Burg et al (4), summarizing the

results of trials carried out at many centres

in the Netherlands, stated that N can be

applied profitably up to 400 kg ha"1 year1 under grazing; and the following advisory

scheme is recommended on mineral soils:

Grazing N, kg ha'1

First 80*-40

Second 80 Third 80 Fourth 60 Fifth 60 Later 40

*Thc high N rate refers to first paddocks to be grazed



GATELY ETAL: NITROGEN FOR MILK PRODUCTION 13

In Northern Ireland, Gordon (5) obtained

426, 623 and 763 cow grazing days ha"1 from the application of 150,300 and 450 kg

N ha"1, respectively.

b) White clover (Trifolium repens): Swards

containing clover respond less to fertiliser

N than do all-grass swards as the clover

supplies some N to the companion grass.

Brockman (6), in a summary of 29 site years

of grazed experiments in England, found

that clover supplied the equivalent of 139

kg N ha"1 with a range of 40 to 220 kg N

ha"1. Ennik (7), using results of research in

the Netherlands and data reported in the

literature, found that "with increasing

application of N, the gain of grass DM is

somewhat higher than the loss of clover

DM in the same mixture, while total DM

yield increases slightly".

c) Stocking rate: Researchers have adopted

different grazing techniques in attempting

to maximise efficiency of conversion of

pasture to animal product and to ascertain

the optimum rate of N that should be

recommended for grazing. With dairy cows

the approach, reported by McMeekan (8),

of using several stocking rates at each level

of N was followed in Ireland by Browne (9, 11) and McFeely (10). It is not practical to

vary the stocking rate during the grazing season with milch cows, apart from bringing in areas that have been used for silage or

hay production. The importance of choos

ing the correct range in stocking rates at the

start of each grazing season is therefore

very critical. The experimental methods for

grazing trials proposed by Conniffe (12) and Connolly (13) have been used in recent

trials by McCarthy and Flynn (14), by Gately (15) and Stakelum (16).

There are two basic assumptions in this

approach: 1) that each treatment has a

critical stocking rate above which milk

output animal1 and stocking rate are

approximately negatively linearly related;

2) that the regression lines describing these linear relationships have a common inter

cept (Fig. 2). Taking any output animal"1 (Y

in Fig. 2) it is shown below that the ratio of the stocking rates giving this output is equal to the ratio of the slopes of the lines. This

ratio is independent of the level of output animal."1

Let X] and x2 be the stocking rates giving a common output animal"1 (Y) for Treat

ments 1 and 2, respectively, and a the

common intercept.

r = #+ _>, *!

Y=a + b2x2 a + bx x}

= a + b2x2

b\/b2 =

xx/x2

The percentage change in carrying capacity of Treatment 2 over Treatment 1, % + C =

100 (_j_,/Z>_2 -

1). Thus, the effect of N is to

increase the carrying capacity by a fixed

percentage C which is determined only by the slope of the regression lines relating

output animal1 and stocking rate.

Clearly fertiliser N recommendations for

the grazing animal must be based mainly on

the results of well-conducted grazing ex

periments. However, in practice, it is

N\x

_ \ *, ro - * -\. E X X. C y _ ,__\ X. L, _ \a TXx

I i\\ a i vr X^^ o , ' \ \

Stocking rate Cx)

Fig. 2: Linear model for two treatments




difficult to design good nitrogen response

trials under grazing which are manageable

due to the number of treatments necessary,

the extent of the land area involved and the

number of animals required. Even when the

results from large-scale grazing trials which

measure N responses are available, great

discretion must be exercised in "generalis

ing" the findings to commercial farming.

Table 1 shows many of the large number of

factors which must be taken into account

when interpreting nitrogen response trials

under grazing. It is obvious that there can

be large variations in the optimum amount

of N required under grazing.

Experimental Six large-scale N X stocking rate grazing

experiments have been conducted in the

Republic of Ireland which used milk pro duction as a measure of the effect of

incremental rates of fertiliser N applied (9, 10, 11, 14, 15, 16). Some details of the soil

groups, texture, drainage characteristics

and sward composition of the sites and

pastures used are shown in Table 2. Three

of the sites (9, 10, 15) were re-seeded

pastures, two (11,14) were old pastures and

one (16) had half of its area sown to a new

pasture and the remainder was an old

pasture. Two of the sites were imperfectly

drained (11, 15) and the remainder were

freely drained.

Clover content

The white clover content of the pastures

varied considerably depending on the site,

year and nitrogen treatment. Table 3 shows

the estimated clover content of the pastures

TABLE 1: Factors, variations in which affect economic optimum use of nitrogen under grazing

Factor group Factor Variable

1. Climate Rainfall, temperature, evapotranspiration Environment 2. Location Aspect, altitude, slope

3. Soil type Soil physical conditions, porosity, carbon content, nutrient

supplying power, rooting depth

4. Drainage, irrigation Type of drainage, irrigation. 5. Pasture type Age and botanical composition of pasture, clover content 6. Stocking rate Number and weight of animals ha"1, age of animals 7. Animal species and breed Cows, cattle, sheep and breed of each

Management 8. Grazing season Date of starting to graze and duration of grazing season 9. Source of nitrogen Ammonium nitrate, urea, etc.

10. Nitrogen application Time and rate of nitrogen over grazing season

11. Previous and current Body condition on going to grass, amount of concentrates

management of animals fed during grazing 12. Grazing system Rotational, set stocking, size of paddocks, length of rest

period 13. Cutting regime Number and size of cuts taken for hay or silage

Economics 14. Costs of inputs and outputs Cost of nitrogen, price of end product such as milk or

beef, interest charges 15. Overheads Cost of land, labour, buildings, roads, fencing etc.




TABLE 2: Soil group, texture, drainage and sward composition of the experimental sites

Trial Soil group Texture Drainage Sward composition result Author

years (reference)

Brown Earth Loam Well drained Re-seeded in 1967, perennial 1971-72 Browne (9)

ryegrass/white clover mixture

Mainly ryegrass and Poa species in 1971-72 with some white clover

Brown Earth Loam Well drained Same site as above but 1973-75 McFeely (10) insignificant clover

Brown Earth Loam Well drained Old pasture with mainly perennial 1978-80 McCarthy and

ryegrass and Poa species, little Flynn (14) clover

Shallow Brown Loam Well drained 50% of area was reseeded pasture 1978-80 Stakelum (16) Earth with perennial ryegrass/white

clover mixture and 50% of area was old pasture with Yorkshire

fog, crested dog's tail, Poa species and white clover

Grey Brown Loam to Well to Old pasture 1967-73 Browne (11) Podzolic silty clay poorly

loam drained Brown Earth Clay loam Well to Re-seeded in 1976-77 with a 1978-80 Gately (15)

Gley (Complex) to loamy imperfectly perennial ryegrass/white clover sand drained mixture

TABLE 3: Estimated mean clover dry matter as a

percentage of total dry matter content on experimen tal sites in lowest N treatment plots in July/August

Author Nitrogen Clover

(reference) Year kg ha ' (%)

Browne (9) 1971-72 58 18

McFeely (10) 1974-75 255 Trace

McCarthy and

Flynn (14) 1978-80 272 Trace

Stakelum(16) 1978-80 146-190 12 Browne (11) 1967-73 58-97 2-1 la

Gately (15) 1978-80 51 15

"Most clover on dry soil area

on the low N plots during July/August which are the peak months for clover

growth in Ireland. Where high N rates were

used, clover contributed insignificantly to

total dry matter production.

Nitrogen rates

The rates ofN applied as calcium ammon

ium nitrate or 'Nitro-chalk' are shown in

Appendix Table 1. The rates varied from 51 to 495 kg ha"1. This includes the N applied for grass conserved for silage, together with

the N used for grazing. Generally 45-50% of the total area was used for first-cut silage and about 30% for second-cut. A dressing of 33-50 kg N ha"1 was applied in January/ February for first grazing and thereafter

these rates of N were applied for every

grazing or every alternate grazing depend

ing on the total amount being used.

Stocking rates

The stocking rates tested are shown in

Appendix Table 1 together with the number of cows in each treatment. They varied

from 1.54 to 3.90 cows ha"1. These stocking rates are on a whole-farm basis. Thus a

stocking rate of 3 cows ha"1 means that

there were 6 cows ha"1 up to late June if

50% of the area was conserved for first-cut




silage, 4.5 cows ha"1 up to early August if

33% ofthe area was conserved for second

cut silage, and 3 cows ha"1 until the end of

grazing in November. Rotational grazing was practised in all experiments with a

grazing period of 3.5 to 2 days and a rest

period of 17.5 to 22 days.

Milk yields The cows calved from January to April

inclusive and the grazing season ranged

from late March to the end of November.

Over 80% of the milk yields shown in

Appendix Table 1 were produced during the grazing season and the remainder from

silage harvested from the experimental area

plus concentrates, except in Gately's experi ment (15) where the milk yields shown were

produced during the grazing season only. Concentrates inputs varied from 0.25 to 0.5

t cow1 and were fed almost entirely while

cows were indoors. Milk yields were re

corded twice daily on at least 5 days per week.

Statistical analysis The percentage changes in carrying capacity were calculated from the results of each

experimental year using the common inter

cept model. This enables sites of obviously

different potentials to be compared on a

common basis. It is the objective of this

study to analyse these percentage changes

and relate them to a number of variables.

Change in carrying capacity may be affected

by a number of factors, e.g. level of applied

N, year of measurement relative to the start

of the experiment, clover content, location

and weather. Because of the relatively low

number of data points it was not possible to

provide a comprehensive analysis for the

effects of all these factors. The effects of

applied N, year within experiment and site

were the variables included in the analysis.

The objective is to develop a model which will be helpful in the analysis of these data.

It must be emphasised that a model

developed and fitted with so few data points and so many possible explanatory variables

must not be interpreted too exactly and can

at best give only a broad picture.

The variables used in the model are:

TVl = lower level of N applied Nu =

higher level of N applied Z =

percentage change in carrying capac

ity observed in an experimental year

in an experiment with N levels Nl and

/Vh.

Y = percentage change in carrying capac

ity in an experimental year with N

levels 0 and N, i.e., the change relative

to zero N.

T = time relative to the start of the

experiment, with the first year being

1. Years 3 and later are given the value

3.

S = site of the experiment. The analysis relies on the following con

siderations. Let

Y=f(N, S,T).1 be the relationship between percentage

change in carrying capacity and N relative

to zero N, site and time. This is illustrated in

Fig. 3. The current analysis attempts to

determine this relationship. Equation 1 will

not apply directly to the change in carrying capacity observed in these experiments

since N -

0 was not a treatment in any

experiment analysed. This is overcome by

the following: In a typical year, Y\ -

f(NL, S, T) and Yn = f(Nu, Sf T) arc the

changes in carrying capacity relative to zero

N for levels Ni and Nn. The observed

percentage change is

z= ioo [(100+ yH)/(ioo+ ro?i] = 100 [(100 +f(Nu, S, T)/( 100 +f(NL, S,

T))-\]....2

Once a functional form is selected for/the




100 +Y

-100+yh-?^r^ 140 - ^? !

o _^**^ I CO_ i

a _^^ CO / I o 130- ^S^

, a, -100+Yl-rrf

I C

yST , |

? 12?" / : z=ioor^?^L-ii

s no- / t ! C / i ? / I

/ '150 1350 100 ^-1-1-L_J-1-1_I_U_L

50 100 NL 200 250 300 NH 400

N level,kg ha-1

Fig. 3: Representation of relationship between % change in carrying capacity (Y) and N

level relative to zero N for a given site and time within experiment

parameters are determined by fitting the

observed values of Z using Equation 2. As

regards the form of/ two conditions must

be considered. First, the response to N

(measured as percentage change in carrying

capacity relative to zero N) would be

expected to be strictly increasing, perhaps after some initial adjustment, and relatively

smooth, starting at the value 0 and rising to

some (perhaps asymptotically attained) upper level. Second, the response and the

upper level may be affected by other factors such as site and time relative to the start of

the experiment.

The curves defined by the form

Y = 100 [K/(\ + A exp (B))?\] .... 3 where A and B are functions of N, site (S) and time (T) were considered. (K would be defined by the boundary condition Y - 0 when N =

0). For certain functional forms

of A and B, this family of curves gives

functions satisfying both conditions out

lined above. However, it must be recognised

that forcing a particular form on the data

for theoretical reasons may result in a fit

that is not quite as good as would be yielded

by a completely empirical approach. On the

other hand, a model may result in fewer,

more readily interpretable parameters.

After a certain amount of preliminary

fitting it seemed best for the purposes of

obtaining functions satisfying both the above conditions to restrict the forms for A

and B as follows:

A = a, a constant, and

B = g (N,S, T) where g is a function of the

three variables in brackets, allowing for

powers and crossproduct terms in these

variables. A number of models of this form

were fitted to the data in Appendix Table 1,

ranging in number of parameters from 4 to

8. The model chosen is shown in Equation 4




TABLE 4: Details of the model which best fitted the data given in Appendix Table I

Model: Y= 100[tf/(l + a exp (jN + k N2 + q T+ r T + v S)) -I]

Where K = 1 + a exp (q T + r T + v Sj

Component Variate Coefficient of variate / value

Constant fo) 0.045 1.26

Experimental (B) N 0.412 0) L04

TV2 -0.479 (k) -3.36 _T 2.332 ra> 3.21

T2 -0.436 <V) -2.68 S -0.560 rv> 3.51

Residual variance = 67.5

Variance ? covariance matrix of coefficients

(a) (N) (N2) (T) (T2) (S) (a) .001278

(N) -.004379 ,156200

(N2) -.001334 ?.054225 .020342

(T) -.024506 .016709 -.001387 .527240

(T2) .005429 ?.007266 .001374 -.117980 .027055

(S) -.000919 .028966 -.008289 -.000053, -.001850 .025501

jV = Nitrogen levels expressed as '00s kg ha'1

T= Time in years of experiment S = Site (freely drained =

1; imperfectly drained = ?1)

Y = % change in carrying capacity

and Table 4. This model had a residual

variance of 67.5 (or residual standard error

of 8.2%). Fitting was by nonlinear estima

tion and hence the / values and variance

covariance matrix must be treated with

some caution. The model selected was

Y = 100[tf/(l + aexp

(jN+krf + qT+rF + vS)) ?1]....4 where N2 and T2 are quadratic terms in N

and T.

Results Table 5 shows the percentage change in

stock-carrying capacity (%+C) due to fer

tiliser N over all sites in the first, second and

third or later years of each experiment. The

percentage change was obtained from re

gression analysis using the common inter

cept model. There was considerable varia

tion in the increases in carrying capacity

due to the higher rates ofN (MO relative to the lower rates (Ni). However, only in 3 out

of the 24 experimental years was there a

negative response to the higher rates of N.

Table 6 shows the mean percentage

change in carrying capacity due to fertiliser

N on the freely drained and imperfectly drained sites. It appears from Table 6 that the percentage change in carrying capacity

was greater on the imperfectly drained than

on the freely drained sites but the N rates

applied were much lower and hence in a

more responsive range on the imperfectly drained sites.

Using the data in Table 5 for freely and

imperfectly drained sites and the model shown in Table 4, predicted responses to a

range of N levels for the first, second and




TABLE 5: Percentage change in stock-carrying capacity (% + C) with nitrogen for the first, second and third or later years of each experiment

Time Nitrogen (kg ha1) Reference Year T M_ Ah %+C No.a

1978 1 146 271 13.5 16

1967 1 58 232 26.4 11

1978 1 51 102 ?3.5 15

1978 1 51 406 18.6 15

1971 1 65 224 19.6 9

1973 1 261 423 5.0 10

1978 1 272 495 ?5.9 14

Average 129 308 10.5 1979 2 190 291 36.1 16

1968 2 58 232 31.6 11 1979 2 51 102 4.8 15

1979 2 51 406 50.8 15 1972 2 65 224 61.6 9

1974 2 255 403 9.6 10

1979 2 272 495 ?2.8 14

Average 135 308 27.4 1980 3 164 287 38.3 16

1969 3 58 232 45.0 U

1970 3 58 232 26.9 11 1971 3 97 224 38.1 II

1972 3 97 224 54.0 11

1973 3 97 224 34.5 10

1980 3 51 102 24.8 15

1980 3 51 406 61.8 15 1975 3 255 403 14.0 10

1980 3 272 495 16.1 14

Average 120 283 35.4

aNo. ll and 15 are imperfectly draining sites

TABLE 6: Mean percentage change in stock-carrying capacity (% +C) on freely and imperfectly drained

sites

Nitrogen (kg ha1) M. Mi % +c

Freely drained 201.5 364.6 18.6

Imperfectly drained 63.8 240.3 31.8

third or later years were calculated and are

given in Table 7.

Figs 4 and 5 are plots ofthe data given in Table 7 for the freely and imperfectly drained sites, respectively. They show that

the predicted response to N in terms of

change in carrying capacity, in these cow

grazing experiments, is much less in the first

year than in later years. On the freely

drained sites, the predicted response to N,

at 300 kg N ha ', compared with zero N, is

28,77 and 89% change in carrying capacity in the first, second and third or later years.

The corresponding predicted change on the

imperfectly drained sites is 16, 44 and 52%,

respectively.

It is apparent from Figs 4 and 5 that the

predicted rate of N which gives the maxi

mum percentage change in carrying capacity is about 300 kg ha"1 on both the freely and




TABLE 7: Predicted percentage change in stock

carrying capacities for a range of N levels relative to zero N for the first, second and third or later years on

the freely and imperfectly drained sites

Experimental year N levels -

(kg ha"1) 1st 2nd 3rd or later

Freely drained sites 0 0 0 0

50 ?2 ?4 ?4 100 2 3 3

200 18 43 49 300 28 77 89 400 30 83 97

500 30 83 97

Imperfectly drained sites 0 0 0 0

50 ?1 ?3 ?3 100 12 2

200 11 27 31 300 16 44 52 400 17 47 55 500 17 48 55

imperfectly drained sites. The shape of the curves in Figs 4 and 5 suggests that there is a decrease in the percentage change in carry

ing capacity below 80 kg N ha"1. Above 300

kg N ha"1 there are only small changes in

carrying capacity due to N. The changes

between 300 and 400 kg N ha1 were only 2, 6 and 8% on the freely drained sites and 1, 3 and 3% on the imperfectly drained sites for the first, second and third or later years,

respectively (Table 7). Although there were not any zero N

treatments in these experiments, it was still

considered desirable to examine the changes

in actual yield cow1 over the years of each

experiment in case there was a consistent

reduction in the low N treatment yields in successive years. If this occurred it would

give larger and larger increases in the

percentage change in carrying capacity due to the high N treatments in each successive year. In general, there was

consistency in the yield changes between

the low and high N treatments in successive

years of each experiment. This indicated

that the time effect (T) shown to be

significant in the model was not due to a

consistent depression of the low N treat

ment yields cow-1 over time.

l . . Year 3 >* ?

g S'*_ -Year 2

S 80- . ^~?? CO ..* y^ ? - S

I 60- // 5 - SY

c 40 - */

a) - // _Year 1

* 20- X/ ,,-''"""' " -

&^~""

0^_ jm^j^_'_i_*_'_i_'_'-1 ^5?^i*'l00 150 200 250 300 350 400 450 500

-10 -

N level,kg ha"'

Fig. 4: Percentage extra stock-carrying capacity from a range of N levels relative to zero N,

on freely drained sites, for the first, second and third or later years




^ 60 -

5 .Year

3

I *

. "-!!1-??-Year 2

? 40 X^^ \ ,<f^ co .?*X^ ? 20

- ..^X c

!x^ -_ ear

| 50 ^^"C' _t _t _< _

o "*^SS^ TsO 200 250 300 350 400 450 500 5? -10 -

N level,kg ha~1

Fig. 5: Percentage extra stock-carrying capacity from a range of N levels relative to zero N,

on imperfectly drained sites, for the first, second and third or later years

Discussion

Four out of the six large-scale experiments included in this review were on freely

drained soils typical of the good dairying land in Ireland. Dairying is also practised on imperfectly to poorly drained land similar to that of the other two sites. The

experiments were carried out over a 12-year

period, the shortest one continued with the

same nitrogen treatments for 2 consecutive

years and the longest one for 4 consecutive

years (Appendix Table 1). The estimated maximum amount of N, at

about 300 kg N ha"1 (Figs 4 and 5), is

considerably larger than the amount used

on most dairy farms at present, although it

is being exceeded by a minority of farmers

on freely drained land. It must be

emphasised that this rate of N is the sum of the amount used for grazing and cutting. In

using fertiliser N for dairying in Ireland, the standard recommendation is, firstly, to use

it for silage, then for first grazing and

thereafter the amount recommended de

pends on the stocking rate. If cows are

indoors for about 150 days, then at a

stocking rate of 3 cows ha"1 it is usual

practice to conserve approximately 50% of

the area for first-cut silage and 33% for

second-cut. These conserved areas receive

120 and 100 kg N ha"1 for first- and second

cut silage, respectively.

The estimated maximum rate of 300 kg N

ha-1 is lower than the 400 kg N ha"1

recommended by Van Burg et al (4) in the

Netherlands where, however, about 150%

of total area is cut for silage compared with

only about 80% in Ireland. The higher removals due to the extra cutting may

account for the increased N requirements in

the Netherlands, particularly as much of

the animal slurry produced during silage

feeding is not returned to the grassland. The

fact that Gordon (5), in Northern Ireland, obtained increased cow-grazing days from

rates ofN up to 450 kg ha"1 could possibly be due to his experimental approach, where

he adjusted the number of cows on each

treatment twice weekly in order to obtain a

constant quantity of herbage on offer. This

interesting experimental technique, al

though not practical in commercial farming,

would optimise herbage use and probably

account for the very large response to N.

The standard error in our model was

8.2%, which, allowing for the variation




introduced by the variation in the parameter

estimates, would lead to 95% confidence

intervals for predicted percentage change

considerably in excess of 17%. This rein

forces earlier comments about the care that

should be taken not to attach too precise an

interpretation to these results.

Effect of clover The parameter of time (T) is not in itself

biologically meaningful and is a surrogate

for perhaps a large number of time-related

processes. Direct measurement of these

processes might explain still further the

unexplained variation in the model. It is

possible that part of this variation could be

accounted for if a more precise measure

ment of the clover content had been

undertaken at each site. Thus, Table 3 gives

only the clover content in July/August

whereas it should have been determined for

each grazing cycle.

The small increase in percentage change

in carrying capacity in the first relative to

the second or subsequent experimental

years from fertiliser N may be due to the

presence of more clover in the first year of

these experiments. There would be a

decrease in clover content with time result

ing from the cumulative effect of N. This

decrease in clover content in the higher N

treatments would be compensated for by

the effect ofthe N on dry matter production

leading to a greater increase in percentage

change in carrying capacity in the second

and subsequent years. This agrees with

TABLE 8: Regression coefficients (b values) of common intercept model for each experimental site

N (kg ha1) Author Year

- lvalues Cows ha.'

(reference)3 NL NH NH at jVh

Stakelum (16) 1978 146 271 ?578 2.40 1979 190 291 ?394 2.40 1980 164 287 ?403 2.40

Browne (11) 1967 58 232 ?1083 2.25 1968 58 232 ?923 2.25 1969 58 232 ?541 2.60 1970 58 232 ?1005 2.60

1971 97 224 ?747 2.82 1972 97 224 ?852 2.82 1973 97 224 ?770 2.82

Gately(15) 1978 51 102 ?907 2.44 1978 51 406 ?738 3.00 1979 51 102 ?666 2.50 1979 51 406 ?463 3.09 1980 51 102 ?377 2.63 1980 51 406 ?290 3.25

Browne (9) 1973 58 224 ?446 3.34 1972 58 224 ?240 3.34

McFeely (10) 1973 261 423 ?927 3.90 1974 255 403 ?895 3.90 1975 255 403 ?502 3.90

McCarthy and 1978 272 495 ?361 3.43

Flynn (14) 1979 272 495 ?280 3.43 1980 272 495 ?622 3.43

aNo. 11 and 15 are imperfectly draining sites




Ennick's (7) conclusions that with increas

ing N usage there is a somewhat greater

gain in grass DM than loss of clover DM.

The suggested decrease in percentage

change in carrying capacity with rates of

fertiliser N up to 80 kg ha"1 (Figs 3 and 4)

agrees with other reports that low levels of

N decrease total production due to its

adverse effect on clover without a com

pensatory increase in grass output (18).

Masterson (19) found that an application of

only 51 kg N ha"1 in spring adversely affected the amount of N fixed by clover in one of these experiments (15).

Effect of soils

Although approximately the same predicted

maximum rate of 300 kg N ha"1 is required for the freely and imperfectly drained sites,

the percentage change in carrying capacity,

due to N, is significantly lower on the wetter

sites (Table 7). In addition Table 8 shows that production animal"1 falls off more

rapidly with increasing stocking rate at the

two wetter sites (11, 15). In practice,

stocking rates are usually considerably

lower on wetter soils due to the shorter

grazing season and the consequent need to

conserve a greater area of the farm for

silage. There is also the risk of more

poaching with increasing stocking rates on

wetter soils.

Overall the results of this review indicate

that dairy farmers in Ireland should not use more than 300 kg N ha"1 on a whole-farm

basis. Because the optimum rate ofN to use

is influenced by so many factors (Table 1) it is probably better in practice to recommend

a range of fertiliser N levels rather than a

single amount. Since our model is based on

the percentage change in stock-carrying

capacity due to N, extra animals would

have overhead costs and this is likely to

affect the rate of N which should be used.

This is particularly so on the wetter soils

where overhead costs are larger than on

drier soils.

Acknowledgments The authors express their sincere thanks to

the research, technician and farm staff who

conducted the experiments reviewed in this

publication at various An Foras Taluntais

Centres. Without their input this review

could not have been undertaken.

We would also like to thank Professor E.

P. Cunningham, Deputy Director, An

Foras Taluntais who suggested this review

and Dr. A. Conway, Head, Johnstown

Castle Research Centre, who encouraged us in this work.

References

1. Murphy, W. E. and O'Keeffe, W. F. "Fertilise use

survey". The Fertiliser Association of Ireland, Dublin, p. 1-15, 1983.

2. Jackson, M. V. and Williams, T. E. Response of

grass swards to fertiliser N under cutting or

grazing,/ agric. Sci., Camb. 92: 549, 1979. 3. Laissus, R. and Jeannin, B. Evolution of grassland

under grazing. Proc. 7th Gen. Meeting Eur.

Grassld Fed., Ghent, p. 5, 35, 1978. 4. Van Burg, P. F. J., Prins, W. E., den Boer, P. J.

and Sluiman, W. J. Nitrogen and intensification of livestock farming in EEC countries. The Fertiliser Society, Proceedings No. 199, 1981.

5. Gordon, F. J. Level of nitrogen fertiliser applied to grassland for dairy cows. European Grassland

Federation, Reading (summaries of papers), 1982. 6. Brockman, J. Quantity and timing of fertilizer

nitrogen for grass and grass clover swards. The Fertiliser Society, Proceedings No. 142, 1974.

7. Ennik, G. C. Grass ? clover interaction especially in relation to N fertilisation. In: "Plant physiology

and herbage production", (edited by C. E.

Wright). Occasional Symposium No. 13, British Grassland Society, 1981.

8. McMeekan, C. P. Grazing management and animal production. Proc. 7th Int. Grassld Congr., Palmerston North, New Zealand, p. 146, 1956.

9. Browne, D. Dairy farming systems. 5th Richards

Orpen Memorial Lecture. Supplement to Ir. Grassld Anim. Prod. Assoc. J. 9: 1, 1974.

10. McFeely, P. Study of nitrogen use for milk

production on free draining land. Anim. Prod.




Res. Rep., An Foras Taluntais, Dublin, p. 95, 1973; p. 108, 1974; p. 109, 1975.

11. Browne, D. Study of nitrogen use for milk

production on permanent pasture at Mullinahone. Soils Res. Rep., An Foras Taluntais, Dublin, p. 94,

1967; p. 85, 1968; p. 59, 1969; p. 62, 1970. Anim. Prod. Res. Rep.r An Foras Taluntais, Dublin, p. 72, 1971; p. 90, 1972, p. 95, 1973.

12. Conniffe, D. Within-herd variance and choice of herd size in grazing experiments. Ir. J. agric. Res. 15: 349, 1976.

13. Connolly, J. The design of grazing experiments. 2. A simple linear model for the gain-stocking rate

relationship. Ir. J. agric. Res. 15: 365, 1976. 14. McCarthy, D. D. and Flynn, F. The effect of

nitrogen and stocking rate on milk production. Proc. Int. Symposium European GrasslandFedera

tion, Wageningen, Pudoc on The role of nitrogen in intensive grassland production, p. 166, 1980.

15. Gately, T. F. Unpublished results. An Foras

Taluntais, Johnstown Castle, Wexford, 1978-80. 16. Stakelum, G. Unpublished results. An Foras

Taluntais, Moorepark Research Centre, 1978-80. 17. Anon. "Monthly Weather Reports". Department

of Transport and Power, Meteorological Service, Dublin, 1931-1960.

18. Maloney, D. and Murphy, W. E. The effect of different levels of nitrogen on a grass clover sward under grazing conditions. Animal output. Ir. J.

agric. Res. 2: I, 1963. 19. Masterson, C. L, and Turner, S. Nitrogen Fixation

in a grazed sward. Soils Res. Rep., An Foras

Taluntais, Dublin, p. 37, 1981.

Received November J I, 1983

APPENDIX

TABLE 1: Nitrogen and stocking rate experiments with milch cows

Mean milk production No. of cows

Nitrogen Stocking rate- treatment"1 Author

kg ha ' cows ha"1 kg cow l kg ha"1 year"1 (reference)

1978 1978 146 1.70 3937 6693 17 Stakelum (16) 156 2.00 3661 7322 20

271 1.90 3881 7374 19 271 2.40 3647 8753 24

1979 1979 190 1.70 3656 6215 17 190 2.00 3647 7294 20

291 1.90 3970 7543 19 291 2.40 3773 9055 24

1980 1980 164 1.70 4054 6892 17 164 2.10 3717 7806 21

287 1.90 4115 7819 19 287 2.40 4017 9641 24

1967 1968 1967 1968 58 1.54 2496a 2875 3844 4428 22 Browne (11) 58 1.76 2558 2903 4502 5109 22

232 1.90 2908 3287 5525 6245 24 232 2.25 2529 2964 5690 6669 24

1969 1969 58 1.65 2678 4419 22 58 2.06 2273 4682 22

232 2.06 2768 5702 24 232 2.60 2555 6643 24




TABLE 1 (continued)

1970 1970

58 1.61 3046 4906 22

58 2.01 2659 5345 22

232 2.09 3137 6556 24

232 2.60 2499 6497 24

1971 1971 97 1.75 3160 5530 22

97 2.25 2875 6469 22

224 2.25 3550 8006 24

224 2.28 2865 8079 24

1972 1973 1972 1973

97 1.75 2921 2915a 5112 5101 22

97 2.25 2397 2905 5393 6536 22

224 2.25 3455 3503 7774 7882 24

224 2.82 2823 3064 7961 8640 24

1978 1978

51 L93 1813a 3499 17 Gately (15) 51 2.51 2016 5060 17

102 2.44 2000 4880 17

406 2.31 2508 5793 17

406 3.00 1999 5997 17

1979 1979

51 1.98 2097 4152 19

51 2.57 1980 5089 19

102 2.50 1980 4950 19

406 2.37 2712 6427 19

406 3.09 2089 6455 19

1980 1980 51 2.08 2371 4932 20

51 2.72 2176 5919 20

102 2.63 2415 6351 20

406 2.50 2742 6855 20

406 3.25 2417 7855 20

1971 1972 1971 1972

58 1.88 3212 3095 6039 5819 18 Browne (9) 58 2.69 2879 2833 7745 7621 18

224 2.42 3259 3310 7887 8010 18

224 3.34 2739 3020 9148 10,087 18

1973 1973

261 2.37 3136a 7432 15 McFeely (10) 261 2.59 3197a 8280 25

261 2.94 3253a 9564 24

261 3.29 3281 10,794 22

423 2.97 3346" 9938 22

423 3.34 3510 11,723 21

423 3.90 2991 11,665 21




TABLE 1 (continued)

1974 1975 1974 1975 255 2.37 3342a 3089a 7921 7320 15 255 2.59 3131" 3211a 8109 8316 25

255 2.94 3197a 3103a 9399 9123 24 255 3.29 3159a 3019 10393 9933 22 403 2.97 34i2a 3262a 10134 9688 22

403 3.34 3398 3225 11349 10771 21 403 3.90 2897 2944 11298 11482 21

1978 1979 1980 1978 1979 1980 1978 1979 1980a

272 2.47 4483 4494 5172 11073 11100 12777 18 19 McCarthy and 495 2.75 4320 4364 5239 II880 12001 14407 20 21 Flynn (14) 272 3.09 4264 4296 4900 13176 13275 15141 18 19 495 3.43 4081 4203 4822 13998 14416 16539 20 21

aData omitted from regression analysis as stocking rate did not reduce production/animal which is essential in order to Fit the common intercept model



review of nitrogen and stocking rate experiments for milk production in ireland

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