Ammonia losses from urine and dung of grazing cattle

PII: S1352–2310(97)00043–5

Atmospheric Environment Vol. 32, No. 3, pp. 295—300, 1998 ( 1998 Elsevier Science Ltd All rights reserved. Printed in Great Britain 1352—2310/98 $19.00#0.00

AMMONIA LOSSES FROM URINE AND DUNG OF GRAZING CATTLE: EFFECT OF N INTAKE SØREN O. PETERSEN,*† SVEN G. SOMMER,* OLE AAES‡ and KAREN SØEGAARD* *Danish Institute of Plant and Soil Science, Research Centre Foulum, DK-8830, Denmark; and ‡Danish Institute of Animal Science, Research Centre Foulum, DK-8830, Denmark (First received 9 October 1995 and in final form 18 June 1996. Published February 1998) Abstract—Nitrogen excretion by cattle during grazing is a significant source of atmospheric ammonia. In this study the relation between NH volatilization and N intake was investigated in wind tunnel experi3 dung pats. Excreta were collected from four groups of dairy cattle ments with simulated urine patches and grazing continuously on either ryegrass fertilized with 300 kg N ha~1 or unfertilized white clover-ryegrass. The two groups of cattle in each grazing system received either 139 or 304 g N cow~1 d~1 in concentrates, corresponding to average total N intakes in the range of 500—700 g N cow~1 d~1. Ammonia losses from dung were insignificant, while total losses from urine, which were estimated by curve-fitting, ranged from 3 to 52% of urinary N. Urea-N in the urine applied in the experiments constituted, with one exception, 64—94% of urinary N. The fraction of urea-N increased significantly with total N concentration in subsamples from individual animals. In the soil, hydrolysis of urea to NH was almost complete within 3 24 h, and release of NH was indicated by scorching. Milk yield and the production of milk protein was not 3 related to N intake or grazing system, while estimated NH losses were significantly reduced at the lower N intake level within the range of N intakes obtained. ( 31998 Elsevier Science Ltd. All rights reserved. Key word index: Wind tunnel, milk protein, milk yield, urea turnover, ryegrass, white clover.

1. INTRODUCTION

In Denmark, dairy cattle are kept grazing on average 4—5 months per year, and during this period excretal returns to the grazing areas may contribute significantly to atmospheric NH (Jarvis, 1993). The volatil3 ization of NH represents an economical loss to the 3 farmer and, when deposited, the NH may disturb 3 natural ecosystems (Pearson and Stewart, 1993). Previous studies have reported losses of urinary N ranging from (5 to 66% (Ball and Ryden, 1984; Lockyer and Whitehead, 1990), while losses from dung pats are comparatively small ((1%) (Ryden et al., 1987). Ryden et al. (1987) suggested an overall emission factor for NH losses from urine of 18%, 3 while a subsequent study by Jarvis et al. (1989b) suggested that this value should be reduced to 11%. Environmental factors like wind speed, relative humidity and temperature influence NH volatilization, 3 but may only partially explain the large temporal variability observed in the field (Hatch et al., 1990). In recent years the Nordic countries have developed a new protein evaluation system with a stronger emphasis on protein quality (Madsen, 1985). The

†Author to whom correspondence should be addressed. Fax: #45 89 99 17 19; E-mail: [email protected].

system attributes a low value for digestible protein to the grass consumed by the cattle which must be supplemented by concentrates and, consequently, during the grazing season total N intakes may be relatively high. The surplus of N is returned mainly as urea in the urine. Urea is rapidly hydrolyzed to NH in the 3 soil and thus at risk of being volatilized (Jarvis et al., 1989a), and an optimized N feeding strategy could therefore limit the potential for NH losses during 3 grazing (Bussink, 1994). In this study the supplementary feed of dairy cows grazing on either white clover-ryegrass or fertilized ryegrass was manipulated to specifically vary the intake of digestible protein. The objective was to investigate whether NH volatilization from grazing cattle 3 could be reduced without a reduction in productivity. 2. MATERIALS AND METHODS

The study was carried out on a well-drained loamy sand (Typic Hapludult) located in central Jutland, Denmark. The soil contained 2.7% C and 0.18%N, and had a pH of 5.5 C!C-2 and a total CEC of 87 meq kg~1. The study involved four different grazing groups with 16 spring-calving dairy cows in each group. The animals were grazed continuously on either perennial ryegrass fertilized with 300 kg N ha~1 or on non-fertilized white clover-ryegrass. The size of the areas was adjusted during the growing season to maintain optimum sward quality, the stock density

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ranging from 3.7 to 8 cows ha~1. The clover percentage increased from ca. 10% to '50% during the 162 d grazing season of 1994 and provided an estimated N input from biological N fixation of 72 kg N ha~1, with an uncertainty of around 15% (F.P. Vinther, unpubl. data). In addition to ad lib. grazing the cattle within each grazing system received either 139 or 304 g N cow~1 d~1 in concentrates to provide different levels of protein balance in the rumen without affecting the calorific value. Both protein levels exceeded the minimum requirements, but were higher and lower, respectively, than the typical level of protein given to high-yielding dairy cows in Denmark (230—250 g N cow~1 d~1). Table 1 presents data on N inputs and returns for the grazing season of 1994. The total inputs from fertilizer, N fixation and concentrates during the grazing season (corrected for the changing size of the grazing area) were 171, 288, 417 and 553 kg N ha~1 for the clover-ryegrass (low and high N) and fertilized ryegrass (low and high N), respectively.

and B) between June 1994 and July 1995 (Table 2). Experiment 1 quantified NH losses from urine and dung collected 3 from cattle of the grass-clover grazing system, i.e. dung (low N), urine (low N), dung (high N) and urine (high N), while Experiment 2 involved urine only from all four grazing groups, i.e. fertilized grass (low N), fertilized grass (high N), grass-clover (low N) and grass-clover (high N). Ammonia in the background air was sampled in duplicate at 25 cm height in front of the wind tunnels. All measurements were duplicated by application of the same treatment to the sward under neighbouring wind tunnels. The wind tunnels used have been described by Olesen and Sommer (1993). Briefly, each unit consisted of a 3]1]0.4 m polycarbonate frame and a steel duct (diameter: 40 cm) equipped with an electric fan, a cup anemometer connected to a datalogger, and six outlets in front of the fan for subsampling the air stream through the tunnel. The wind speed inside the steel duct was maintained at 3—3.5 m s~1. The air sampled was combined and led through a single acid trap per tunnel, where NH was absorbed in ca. 80 ml 3 20 mM H PO , and then through a gas meter. Acid traps 3 4 were typically replaced after 6 h, and then after 1, 2, 3, 6, 10 and (Experiment 2) 14 d; background values were subtracted for each period. Urine and dung was collected overnight using urinals and trays from two animals per treatment (Experiment 1), or

2.1. Experimental Ammonia volatilization from simulated dung pats and urine patches was measured outside the grazed plots using eight wind tunnels. The sward was cut to ca. 5 cm height in front of the wind tunnels prior to the two experiments (1 and 2) which were each carried out on two different occasions (A

Table 1. A total balance for the 162 d grazing season of 1994 for cattle grazing on either white clover—ryegrass or on fertilized ryegrass and receiving either 139 (low N) or 304 g N cow~1 d~1 in concentrates. The inputs are given on an area basis, while N intake and returns are given per head, since the stock density was adjusted during the growing season to optimize herbage quality and allowance Clover-ryegrass Units Inputs Fertilizer N fixation Concentrates N intake! Returns Milk yield" Milk protein Urine!,# Dung!,#

Fertilized ryegrass

Low N

High N

Low N

High N

kg N ha~1 kg N ha~1 kg N ha~1 kg N cow~1

— 72 99 89

— 72 216 117

300 — 117 83

300 — 253 116

kg cow~1 kg N cow~1 kg N cow~1 kg N cow~1

4277 23 46 (29) 22 (13)

4082 23 72 (47) 22 (15)

4309 23 41 (26) 19 (13)

4504 24 70 (46) 22 (14)

! Calculated (see text). " Energy corrected. # Estimated field excretion in parentheses.

Table 2. Ammonia volatilization from dung and urine (Experiments 1A and 1B) or from urine only (Experiments 2A and 2B) applied to a cut sward was quantified using wind tunnels. Both experiments were carried out on two different occasions as indicated. The table presents the sources and N concentrations (g N kg~1) of the applied excreta. Urea-N as a percentage of urinary N is shown in parentheses Clover-ryegrass Low N Experiment 1A 1B 2A 2B

Period 21 26 20 20

June — 1 July 1994 July — 5 August 1994 September — 4 October 1994 June — 4 July 1995

Dung 3.5 3.4

Urine 5.0 (27)! 7.6 (70) 11.0 (84) 6.2 (64)

Fertil. ryegrass

High N Dung 4.6 5.0

Urine 11.2 9.5 13.3 11.7

Low N

High N

Urine

Urine

(80) (83) (94) 10.0 (70) (86) 6.1 (71)

10.9 (87) 4.8 (79)

!The cattle were recovering from a period of protein deficiency due to poor clover growth during the early spring.

Ammonia losses from grazing cattle collected in buckets during urination from 8 to 12 animals per treatment at the times of milking (Experiment 2). In connection with Experiment 2B (June 1995), subsamples of urine from individual animals were analyzed for urea-N and total-N. The excreta were stored at 2°C for a maximum of 5 days before use in the wind tunnel experiments; during this time (1% of the urea was hydrolyzed (data not shown). Dung was applied at a rate of 2.55 kg on a 0.05 m2 area, while urine (2.55 kg) was distributed on a 0.6 m2 area using a watering can. The N concentrations of the excreta are presented in Table 2. In parallel with Experiment 1B, the turnover of urea-N and pH in the soil was studied in a urine patch established close to the wind tunnels. The urine patch was protected from precipitation by a transparent cover to make these data comparable with the NH loss measurements. Here five randomly located soil cores3 (diam., 20 mm) were sampled to 10 cm depth at the times indicated above, and divided into 0—2, 2—5, and 5—10 cm depth intervals. The soil was transported in an insulated box to a 2°C storage room, where roots were removed and sampling locations mixed. Duplicate 5 g fresh weight soil samples were extracted in 25 ml 1 N KCl for 30 min, centrifuged at 2°C, filtered, and then the extracts were stored at !20°C until analyzed. Soil samples were dried at 105°C for 24 h to determine the soil water content. 2.2. Analytical techniques Ammonia was determined colorimetrically using a QuickChem 4200 flow injection analyzer (Lachat Instr., Milwaukee, Wisconsin, U.S.A.). Urea-N was determined with the diacetyl monoxime method as modified by Mulvaney and Bremner (1979). Total-N was measured on a Kjeltec 1030 Analyzer (Tecator, Ho¨gana¨s, Sweden). Milk protein was analyzed on a Milko-Scan 104 (Foss Electric, Hillerød, Denmark). 2.3. Calculations Nitrogen intake and excretion by the cattle were calculated on the basis of N concentrations in herbage and supplementary feed. The N concentration of the herbage was determined every 2—4 weeks, and for the weeks where excreta were collected the N concentration was calculated by interpolation. Herbage intake was estimated from the theoretical energy requirements for milk production, live weight changes and maintenance minus the intake of concentrates, and using an energy utilization efficiency of 87% (Kristensen and Jensen, 1989). Milk protein and milk yield was determined once a week per animal. The curve shape of accumulated NH losses suggested 3 that frequently some volatilization occurred after the experiments were terminated. The total losses were therefore estimated by fitting the logistic equation presented by Demeyer et al. (1995) to the data:

297

ation (Whitehead and Raistrick, 1991), and Sommer et al. (1991) found that the loss increased with wind speed up to 2.5 m s~1. Hence, the NH losses ob3 served probably represent potential, rather than actual losses inside the grazed plots. Figure 1 illustrates the sigmoidal pattern of NH3 volatilization from urine patches with maximum loss rates occurring 1—2 days after application (data from Experiments 1A and 1B; time course data for Experiments 2A and 2B are not presented). Total NH3 losses (N.!9) from urine patches were extremely variable in this study, ranging from 3 to 52% of urinary N, the highest losses occurring in Experiment 2A (September 1994). Losses of NH3 from dung pats (Fig. 1) were insignificant, as shown in the previous studies (Kellems et al., 1979; Ryden et al., 1987). It is likely that the formation of a surface-crust limited NH3 volatilization, before any significant mineralization of N in the dung occurred. The N concentration of individual urine samples showed large variations within each treatment (Fig. 2) with coefficients of variation of 20—44% (n*8), as well as a diurnal pattern with higher concentrations in the morning. This concurs with observations made by Betteridge et al. (1986) on two steers during three 24 h periods. The fraction of urea-N increased significantly with total N in the urine (P(0.001, n"39). Hatch et al. (1990) examined the relationship between NH3 losses from grazing cattle and wind speed, soil and air temperature, air humidity, evapotranspiration and precipitation, but could only account for 40% of the variation. Part of the remaining variability may be related to the observed variability with respect to urine composition.

F(t)"N

(1!e(~ct))i (1) .!9 where F(t) is the accumulated NH loss (g N), N is the 3 .!9 maximum loss at t"R (d), c is a rate constant '0, and i is related to the point of inflection of the curve. Curve fitting was carried out using SigmaPlot 2.0 (Jandel Corp., 1994). According to this procedure the measured fluxes of NH 3 underestimated total losses by !5 to 30% (mean$SD, 8$9%).

RESULTS AND DISCUSSION

The use of wind tunnels for quantification of NH 3 volatilization excluded the effect of precipitation, while wind speeds inside the tunnels were maintained at 3—3.5 m s~1. Precipitation may reduce volatiliz-

Fig. 1. Ammonia losses from urine patches and dung pats in wind tunnel experiments. Excreta were collected from cattle grazing continuously on white clover-ryegrass pastures and receiving either 139 (low N) or 304 g N cow~1 d~1 (high N) in concentrates. Symbols show accumulated losses in Experiment 1A (A) and Experiment 1B (B) expressed as percentages of total N applied, while dotted lines are curves fitted to the measured data to obtain an estimate of total NH losses 3 (N ) from urine patches. In Experiment 1B one of the dung .!9 treatments was disregarded due to contamination of the gas sampling system.

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Fig. 2. Concentrations of urinary N (g kg~1) and the fraction of urea-N measured in subsamples of urine from individual cows grazing on white clover-ryegrass. The cows received 304 (high N) or 139 g N cow~1 d~1 (low N) in concentrates. The subsamples were obtained during collection of urine for Experiment 2B.

Fig. 3. The turnover of urea in the soil was studied in a separate urine patch outside the grazed areas in parallel with Experiment 1B. Five soil cores were randomly sampled, sectioned, and the depth intervals 0—2 cm, 2—5 cm and 5—10 cm pooled and analyzed for concentrations of urea (A) and NH` (B) at the times indicated (n"2). 4

The urea was almost completely hydrolyzed, in the soil within 24 h after application and thus at risk of being volatilized (Fig. 3A). It indicates that an increasing fraction of urea-N in the urine will stimulate NH 3 losses, and a relationship was in fact suggested between the percentage of urinary N lost via NH vola3 tilization and the fraction of urea-N in the urine (P"0.07, n.s.). Exchangeable NH` in the soil (Fig. 4 3B) remained at a high level in the top 2 cm until day 3, although an upward flux was indicated by the NH 3 volatilization measurements, and a downward flux by increasing NH` concentrations at 2—5 and 5—10 cm 4

depth. Scorching may have replenished the NH` pool 4 by damaging roots of the ryegrass (Richards and Wolton, 1975). Above-ground scorching of clover was always more severe than scorching of the ryegrass in this study. Lantinga et al. (1987) reported that scorching was related to the urine N concentration, and scorching thus provides a mechanism that, like the fraction of urea-N in urine, may stimulate NH vola3 tilization above the level expected from the urinary N concentration alone. The N utilization efficiency of intensively managed grassland systems with dairy cattle is typically 15—20%, which is less than half of the theoretical maximum efficiency of 40—45% (Vuuren and Meijs, 1987; Valk and Hobbelink, 1992). Vuuren and Meijs (1987) found that replacing fresh herbage with a mixture of fresh herbage and maize silage, thereby reducing N intake from 626 to 494 g N cow~1 d~1, increased the efficiency from 17 to 24% without affecting milk production. The strategy is to replace fresh herbage, which is excessively degraded in the forestomachs, with feeds that are either converted to microbial protein in the rumen or transported to the small intestine and directly utilized by the cow. In the present study the composition of concentrates was manipulated to vary the intake of digestible protein, but without restricting the access to grazing. Total N intakes averaged 505—715 g N cow~1 d~1 during the grazing season; the lower limit corresponds to the typical annual N input for Dutch dairy cows (Tamminga, 1992). In 1994 the N utilization efficiency (milk protein N vs total N intake) was ca. 20% at the high N level and ca. 27% at the low N level in both grazing systems. Table 3 presents the total N intake (NI), production of milk protein (MP) and, in parentheses, the concentration of herbage N for the weekly periods where urine and dung for the NH loss measurements 3 were collected. The variations of NI between experiments reflected the N concentration of the sward which reached almost 5% in the clover-ryegrass (high N) treatment in September 1994, mainly due to an increasing clover percentage. The MP yield dropped during the grazing season due to the stage of lactation (data not shown). Both the clover percentage and the reduced MP yield probably stimulated NH losses via 3 increased N excretion in the last part of the grazing season. Figure 4 shows MP (A) and NH losses (B), respec3 tively, as a function of NI for the weekly periods covered in this study. Ammonia losses were estimated from calculations of urinary N excretion and corrected for the time not grazing; the actual percentage losses of the individual experiments were used. There was no difference (P'0.5, n"6) in MP between the high N and low N treatments, whereas NH volatiliz3 ation was significantly (P"0.014, n"6) related to the N intake level, as determined by a paired-sample t-test (Zar, 1984). The average decrease in emission rate at the low N intake level was 55%. Emissions

Ammonia losses from grazing cattle

299

Table 3. Nitrogen intake (NI) and production of milk protein (MP) (g N cow~1 d~1) by cattle grazing at low or high N regimes on clover-ryegrass or fertilized ryegrass (paired data for the weekly periods where excreta for the NH 3 volatilization experiments were collected). The concentrations of N (%) in the herbage are given in parentheses Clover-ryegrass Low N Experiment

Week!

1A

20—26 June 1994

1B

25—31 July 1994

2A

19—25 September 1994

2B

12—18 June 1995

Fertilized ryegrass

High N

NI

MP

NI

MP

516 (2.7) 526 (4.1) 614 (4.9) 574 (3.9)

142

717 (3.4) 666 (3.9) 803 (4.9) 778 (3.5)

158

143 112 154

Low N

High N

NI

MP

NI

MP

598 (4.1) 505 (3.1)

119

763 (4.1) 754 (3.0)

119

130 108 166

140

145

! The weekly periods where urine and dung were collected.

Fig. 4. Total N intake, production of milk protein and NH 3 volatilization calculated for the weeks where urine and dung were collected. Ammonia losses were estimated from the excretion of urine, corrected for the time not grazing, and the actual percentage losses determined in the wind tunnel experiments. The figure shows the relationship between N intake and, respectively, milk protein (A) and NH losses (B); 3 the symbols refer to the experiment (see Table 2) and the extention (H"high or L"low) to the level of N intake. The two grazing systems (clover-ryegrass and fertilized ryegrass) behaved similarly and are not distinguished in the figure.

from the clover-ryegrass system were similar to those from fertilized grass, although total N inputs to the clover-ryegrass system were considerably lower (see Table 1). On an area basis NH volatilization ranged 3 from 20 to 570 g N ha~1 d~1, which is in good agreement with the range observed by Jarvis et al. (1989b) in a similar study with yearling steers grazing on white clover-ryegrass or ryegrass fertilized with 210—420 kg N ha~1, where NH losses were quantified with a mi3 crometeorological technique. Jarvis et al. (1989b) suggested an overall emission factor of 11% for urinary N deposited by grazing cattle. The losses recorded in the present study cannot be extrapolated in time to give an estimate of NH volatilization for the entire 3 grazing season without more detailed information about the importance of and possible interactions between herbage N content, N excretion, and urinary N composition.

Smits et al. (1995) compared NH emissions from 3 housed dairy cattle receiving alternately two diets with a difference in rumen-degradable protein corresponding to 160 g N; at the lower level of N intake the emission rate was reduced by 40%. The results of the present study suggest that there is also a potential for reducing NH losses from dairy cattle during grazing 3 and that, within the range of N intakes covered in this study, this can be obtained without adverse effects on productivity. The grassland management is important for the total balance of digestible protein, and the conclusions of this study should not be extended to situations with limited access to grazing or with suboptimal herbage quality. Acknowledgement—The technical assistance of Lene Skovmose is greatly appreciated.

REFERENCES

Ball P. R. and Ryden J. C. (1984) Nitrogen relationships in intensively managed temperate grasslands. Plant Soil 76, 23—33. Betteridge K., Andrewes W. K. G. and Sedcole J. R. (1986) Intake and excretion of nitrogen, potassium and phosphorus by grazing steers. J. agric. Sci. Camb. 106, 393—404. Bussink D. W. (1994) Relationships between ammonia volatilisation and nitrogen fertiliser application rate, intake and excretion of herbage nitrogen by cattle on grazed swards. Fert. Res. 38, 111—121. Demeyer P., Hofman G. and Van Cleemput O. (1995) Fitting ammonia volatilization dynamics with a logistic equation. Soil Sci. Soc. Am. J. 59, 261—265. Hatch D. J., Jarvis S. C. and Dollard G. J. (1990) Measurements of ammonia emission from grazed grassland. Envir. Pollut. 65, 333—346. Jarvis S. C. (1993) Nitrogen cycling and losses from dairy farms. Soil ºse and Manage. 9, 99—105. Jarvis S. C., Hatch D. J. and Roberts D. H. (1989a) The effects of grassland management on nitrogen losses from grazed swards through ammonia volatilization; the relationship to excretal N returns from cattle. J. agric. Sci. Camb. 112, 205—216.

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Jarvis S. C., Hatch D. J. and Lockyer D. R. (1989b) Ammonia fluxes from grazed grassland: annual losses from cattle production systems and their relation to nitrogen inputs. J. agric. Sci. Camb. 113, 99—108. Kellems R. O., Miner J. R. and Church D. C. (1979) Effect of ration, waste composition and length of storage on the volatilization of ammonia, hydrogen sulfide and odors from cattle waste. J. Animal Sci. 48, 436—445. Kristensen E. S. and Jensen M. (1989) Utilization of grassland for dairy production — management and performance. In Research in Cattle Production Systems (edited by Østergaard V. and Hindhede J.). Report No. 661, pp. 15—53. The Danish Institute of Animal Science, Res. Ctr. Foulum, DK-8830 Tjele. Lantinga E. A., Keuning J. A., Groenwold J. and Deenen P. J. A. G. (1987) Distribution of excreted nitrogen by grazing cattle and its effects on sward quality, herbage production and utilization. In Animal Manure on Grassland and Fodder Crops (edited by v. d. Meer H. G. et al.), pp. 103—117. Martinus Nijhoff Publishers, Dordrecht. Lockyer D. R. and Whitehead D. C. (1990) Volatilization of ammonia from cattle urine applied to grassland. Soil Biol. Biochem. 22, 1137—1142. Madsen J. (1985) The basis for the proposed Nordic protein evaluation system for ruminants. The AAT-PBV system. Acta Agric. Scand. 25, (Suppl). 9—20. Mulvaney R. L. and Bremner J. M. (1979) A modified diacetyl monoxime method for colorimetric determination of urea in soil extracts. Commun. Soil Sci. Plant Anal. 10, 1163—1170. Olesen J. E. and Sommer S. G. (1993) Modelling effects of wind speed and surface cover on ammonia volatilization from stored pig slurry. Atmospheric Environment 27A, 2567—2574.

Pearson J. and Stewart G. R. (1993) The deposition of atmospheric ammonia and its effects on plants. New Phytol. 125, 283—305. Richards I. R. and Wolton K. M. (1975) A note on urine scorch caused by grazing animals. J. Br. Grassl. Soc. 30, 187—188. Ryden J. C., Whitehead D. C., Lockyer D. R., Thompson R. B., Skinner J. H. and Garwood E. A. (1987) Ammonia emission from grassland and livestock production systems in the UK. Envir. Pollut. 48, 173—184. Smits M. C. J., Valk H., Elzing A. and Keen A. (1995) Effect of protein nutrition on ammonia emission from a cubicle house for dairy cattle. ¸ivestock Prod. Sci. 44, 147—156. Sommer S. G., Olesen J. E. and Christensen B. T. (1991) Effects of temperature, wind speed and air humidity on ammonia volatilization from surface applied cattle slurry. J. agric. Sci. Camb. 117, 91—100. Tamminga S. (1992) Nutrition management of dairy cows as a contribution to pollution control. J. Dairy Sci. 75, 345—357. Valk H. and Hobbelink M. E. J. (1992) Supplementation of grazing dairy cows to reduce environmental pollution. Proc. 14th Gen. Meet. European Grassland Fed., Lekti, Finland, pp. 400—405. Vuuren A. M. v. and Meijs J. A. C. (1987) Effects of herbage composition and supplement feeding on the excretion of nitrogen in dung and urine by grazing dairy cows. In: Animal Manure on Grassland and Fodder Crops (edited by v. d. Meer H. G. et al.), pp. 17—25. Martinus Nijhoff Publishers, Dordrecht. Whitehead D. C. and Raistrick N. (1991) Effects of some environmental factors on ammonia volatilization from simulated livestock urine applied to soil. Biol. Fertil. Soils 11, 279—284. Zar J. H. (1984) Biostatistical Analysis, 2nd Edn. PrenticeHall, London, U.K.