biomass production and nutrient removal by tropical ...rsyost/v-gicaporteryostbiomass.pdf ·...

Biomass production and nutrient removal by tropicalgrasses subsurface drip-irrigated with dairy effluent

R. B. Valencia-Gica, R. S. Yost and G. Porter

Department of Tropical Plant and Soil Sciences, University of Hawaii at Manoa, Honolulu, HI, USA

Abstract

Effluent lagoons on dairy farms can overflow and

potentially pollute adjacent land and associated water

bodies. An alternative solution to effluent disposal is

needed by dairy operators in island environments. An

attractive win-win alternative is to recycle nutrients

from this resource through effluent irrigation for forage

grass production that minimizes environmental pollu-

tion. This study assessed biomass production and

nutrient removal by, and high application rates to,

tropical grasses that were subsurface drip-irrigated with

dairy effluent. Four grass species – Banagrass (Pennise-

tum purpureum K. Schumach.), California grass

(Brachiaria mutica (Forssk.) Stapf.), Stargrass (Cynodon

nlemfuensis Vanderyst) and Suerte grass (Paspalum atra-

tum Swallen) – were subsurface (20–25 cm) drip-

irrigated with effluent at two rates based on potential

evapotranspiration (ETp) at the site (Waianae, Hawaii)

)2Æ0 ETp (16 mm d)1 in winter; 23 mm d)1 in summer)

and 0Æ5 ETp (5 mm d)1 in winter; 6 mm d)1 in sum-

mer). Treatments were arranged in an augmented

completely randomized design. Brachiaria mutica and

P. purpureum had the highest dry-matter yield (43–

57 t ha)1 year)1) and nutrient uptake especially with

the 2Æ0 ETp irrigation rate (1083–1405 kg ha)1 year)1

N, 154–164 kg ha)1 year)1 P, 1992–2141 kg ha)1

year)1 K). Average removal of nutrients by the grasses

was 25–94% of the applied nitrogen, 11–82% of

phosphorus and 2–13% of the potassium. Average

values of crude protein (90–160 g kg)1), neutral deter-

gent fibre (570–620 g kg)1) and acid detergent fibre

(320–360 g kg)1) were at levels acceptable for feeding

to lactating cattle. Results suggest that P. purpureum and

B. mutica irrigated with effluent effectively recycled

nutrients in the milk production system.

Keywords: dry-matter production, nutrient uptake,

dairy effluent irrigation, crude protein, acid detergent

fibre, neutral detergent fibre, Hawaii

Introduction

Livestock and dairy production is often a concentrated

animal-intensive operation, especially in an island

environment. Such enterprises rely mostly on imported

feeds to sustain their operation, which results in a

tremendous influx and accumulation of nutrients and

salts. These nutrients and salts can cause pollution of

land and water bodies receiving the effluent. If the

animal wastes are not re-utilized or exported, an open

nutrient cycle is created in the milk production system.

An attractive alternative for dairy producers is to apply

the effluent for forage production to recycle the

nutrients. Water and nutrients contained in dairy

effluent may, in this way, be used to maintain or

improve soil fertility and enhance forage production

and quality. Reapplying animal effluent for forage

production improves the efficiency of feed sourcing in

dairy production and minimizes environmental pollu-

tion.

Various grasses and crops have been used to remove

nitrogen (N) and phosphorus (P) from wastewater, for

example, Stargrass (Cynodon nlemfuensis), Bermudagrass

(Cynodon dactylon (L.) Pers.), Johnson grass (Sorghum

halepense (L.) Pers) (McLaughlin et al., 2004; Adeli et al.,

2005; Valencia et al., 2006) and Bahiagrass (Paspalum

notatum Flugge) (Adjei and Rechcigl, 2002). Various

forage systems have also been tested for the effects of

dairy effluent or slurry application on nutrient removal,

crop yield and forage quality (e.g. Macoon et al., 2002;

Woodard et al., 2002).

Many tropical grass species have high biomass pro-

ductivity, high nutrient removal potential, acceptable

nutrient quality, persistence, tolerance to harsh envi-

ronmental conditions, resistance to pests and diseases

and ease of establishment (Takahashi et al., 1966; Duke,

Correspondence to: R. B. Valencia-Gica, Department of

Tropical Plant and Soil Sciences, c ⁄ o R.S. Yost, University

of Hawaii at Manoa, Honolulu, HI 96822, USA.

E-mail: [email protected]

Received 10 December 2010; revised 6 September 2011, 2 November

2011

doi: 10.1111/j.1365-2494.2011.00846.x � 2012 Blackwell Publishing Ltd. Grass and Forage Science 1

1983; Pant et al., 2004). In Hawaii, Pennisetum purpur-

eum produced up to 336 t ha)1 year)1 of green forage

(Takahashi et al., 1966) or approximately 70 t ha)1

year)1 of dry matter, which is close to the 85 t ha)1

year)1 reported for P. purpureum that received

897 kg N ha)1 year)1 and was cut every 90 d under

rainfall conditions of nearly 2000 mm per year (Vicen-

te-Chandler et al., 1959). Given its high biomass and

fixed carbon yield, P. purpureum was considered a

promising feedstock for the production of metallurgical

charcoal (Yoshida et al., 2008).

In recent years, there has been a renewed interest in

the use of wastewater for growing crops or trees, or

irrigating golf courses and turf grasses, all motivated by

environmental objectives. One study on wastewater

reuse conducted in Hawaii to remove N from second-

arily treated domestic sewage effluent used Brachiaria

mutica (Handley and Ekern, 1981). The relatively high

consumptive water use (4 mm d)1) of this grass cou-

pled with its fast and thick growth led to removal of

about 69% of the effluent N at application rates of 475–

2600 kg N ha)1 year)1. While some studies have inves-

tigated the use of subsurface drip irrigation for applying

animal effluent (Stone et al., 2008; Cantrell et al., 2009),

to our knowledge, no study has determined the

potential of tropical grasses with subsurface drip irriga-

tion. This study aims to assess the nutrient uptake,

productivity and forage quality of four tropical forages

subsurfaces, drip-irrigated with dairy effluent based on

the potential evapotranspiration (ETp).

Materials and methods

Soil and climate

The experimental site was in Waianae (21�26¢N ⁄158�10¢W), O’ahu, Hawaii, with a mean elevation of

2Æ4 m above sea level and average monthly ETp ranging

from 5 to 21 mm d)1, varying with season. Rainfall is

very low (150 mm annually), and the ETp is, thus, high

compared with other locations on the island (Valencia-

Gica et al., 2010). Rainfall, ETp and other weather data

were collected using the Hobo� (Onset Computer

Corporation, Bourne, MA, USA) weather station

installed at the site. The ETp was calculated using the

Reference ETp Calculation and Software (Ref-ET v. 2.0),

which uses the Penman equation (Allen, 2001).

Monthly average ETp was obtained from the daily ETp

calculated by Ref-ET.

The soil (Pulehu series) is a fine-loamy, mixed,

semiactive, isohyperthermic Cumulic Haplustoll (Soil

Survey Staff, 1972) and has 630 g kg)1 clay, 190 g kg)1

silt and 180 g kg)1 sand. Soil pH was already initially

high (ranging from an average of 7Æ5–7Æ6 in July 2003),

and with continuous irrigation with high pH effluent

(ranging from 7Æ9 to 8Æ8), the soil pH further increased

(ranging from an average of 8Æ3–8Æ5) after 2 years.

Experimental design

Twelve plots (which are the experimental units), each

measuring 13Æ4 m2, were planted in December 2003 to

four tropical grass species that were observed to tolerate

saline soil in Waianae (ECsaturated paste extract 1:2 soil-water ratio =

11–26 dS m)1). These grasses included: Banagrass

(P. purpureum K. Schumach. cv. HA 5690), California

grass (B. mutica (Forssk.) Stapf.), Stargrass (C. nlemfuen-

sis Vanderyst) and Suerte grass (Paspalum atratum

Swallen cv. Suerte). Pennisetum purpureum is a variety

of elephant grass. Brachiaria mutica is also called para-

grass. These species are relatively untested in this high-

solar-radiation environment, and it was a major objec-

tive to investigate how they performed with applica-

tions of dairy lagoon effluent in a tropical environment

such as that in Hawaii. The grasses were allowed to

establish to 95 to 100% ground cover with freshwater

irrigation (30 min, twice a day) until July 2004.

Treatments were arranged in an augmented com-

pletely randomized design, patterned after the aug-

mented block design developed by Federer (1956,

2005). Two rates of effluent irrigation were applied to

P. atratum and P. purpureum with two replications each

(Figure 1). Augmented treatments (single replications)

were B. mutica (planted in January 2005) and C. nlemf-

uensis (planted at the beginning of the experiment) that

also received two rates of effluent irrigation. Irrigation

rates were based on the ETp at the site: 2Æ0 ETp (average

of 16 mm d)1 in winter and 23 mm d)1 in summer)

and 0Æ5 ETp (average of 5 mm d)1 in winter and

6 mm d)1 in summer). From August 2004 to 2006,

plots were irrigated accordingly through a subsurface

(20–25 cm depth) drip effluent irrigation system. For-

age yield, quality and soil chemical measurements were

taken every 4–6 weeks during the 2-year experiment.

The emphasis on temporal measurement and change

permitted testing the hypothesis of no change in forage

production, forage quality and soil properties over time

with effluent irrigation. The number of replications was

limited by land availability and to provide more robust

data on repeated measurement of forage and soil

properties over time. Additional information on exper-

imental design and properties of dairy effluent was

presented in an associated article (Valencia-Gica et al.,

2010).

Micronutrients such as iron (Fe), manganese (Mn),

zinc (Zn) and copper (Cu) were applied as foliar

fertilizer beginning in April 2005 (Zn), November

2005 (Mn, Fe) and May 2006 (Cu) and continued for

the duration of the experiment. The amount of

micronutrients applied was adjusted every month

2 R. B. Valencia-Gica et al.

� 2012 Blackwell Publishing Ltd. Grass and Forage Science

depending on the results of the tissue analyses. Appli-

cation rates were 337–562 g ha)1 month)1 for Zn, 56–

337 g ha)1 month)1 for Cu, 337–562 g ha)1 month)1

for Fe and 562–1123 g ha)1 month)1 for Mn.

Nutrient content of dairy effluent was analysed peri-

odically for estimating nutrient loads (Figure 2). At the

2Æ0 ETp rate, cumulative nutrient loads were estimated at

1200 kg N ha)1 year)1, 620 kg P ha)1 year)1, 42 000

kg K ha)1 year)1, 2900 kg Ca ha)1 year)1 and 5900 kg

Mg ha)1 year)1. At the 0Æ5 ETp rate, cumulative nutrient

loads were estimated at 370 kg N ha)1 year)1, 190 kg P

ha)1 year)1, 12 000 kg K ha)1 year)1, 850 kg Ca ha)1 -

year)1 and 1700 kg Mg ha)1 year)1.

Grass harvesting

Grasses were well established prior to a first harvest

in August 2004. Pennisetum purpureum and P. atratum

were initially harvested manually at 30 cm height

(August 2004–October 2004), whereas B. mutica and

Brachiaria mutica

(2 0 ETp)

Pennisetum purpureum (0 5 ETp)

Paspalum atratum (2 0 ETp)

Pennisetumpurpureum (0 5 ETp) (2·0 ETp)(0·5 ETp)(2·0 ETp)(0·5 ETp)

CheckCheckvalve

·66

m

Cynodonnlemfuensis

(0·5 ETp)

Cynodonnlemfuensis

(2·0 ETp)

Paspalumatratum

(0·5 ETp)

Pennisetumpurpureum (2·0 ETp)

3·

DairyLagoon

( p) ( p)( p) ( p)

3·66 m0·3 m

Lagoon

Paspalumatratum

(2 0 ETp)

Pennisetumpurpureum (2 0 ETp)

Freshwater source

(2·0 ETp)(2·0 ETp)

1·7 cm Ball valve

Vacuum breaker

2·54 cm Solenoid valve 2·0 ETp

2·54 cm Solenoid valve

2·54 cm PVC supply line

0·5 ETp

Paspalumatratum

(0·5 ETp)

Brachiariamutica

(0·5 ETp)

Headwork shelter

2·54 cm PVC FW flush line

( p) ( p)

2·54 cm PVC supply line

Figure 1 Experimental treatment layout for tropical grasses drip-irrigated with dairy effluent, Waianae, O’ahu, Hawaii.

Figure 2 Nutrient concentrations in dairy effluent used for irrigation of tropical grasses in Waianae, O’ahu, Hawaii, June 2004–

August 2006.

Productivity of effluent-irrigated grasses 3


C. nlemfuensis had been cut using a sickle mower at 8 cm

height. From November 2004 to August 2006, all

grasses were cut using a sickle mower at the same

height of 8 cm to simulate expected dairy farmers’

practice. A 0Æ30 m border area on each side of the

experimental unit was excluded from the yield calcu-

lations, resulting in an effective biomass harvest area of

9Æ29 m2 per plot. The harvesting interval in the first year

(August 2004–July 2005) was 27–29 d. This frequent

harvesting was intended to provide high-quality forage.

Growth progression of the grasses was measured from

August to October 2005. During the second year

(August 2005–2006), the harvesting interval was

extended to 6 weeks based on the first year’s results

and occurred after 42–44 d of growth. During each

harvest, soil and soil-solution samples were also col-

lected and analysed for nutrients (Valencia-Gica, 2007).

Results on soil phosphorus changes were presented in

an associated article (Valencia-Gica et al., 2010), and

salinity changes will be discussed subsequently.

Plant tissue analyses

Subsamples were taken from each plot’s harvest and

dried in a forced-air oven. Annual biomass production

and nutrient uptake (average of the 2-year annual data)

were calculated from the harvest and tissue nutrient

data. Nutrient removal as a percentage of the applied

nutrient was calculated as the nutrient uptake mea-

sured from the harvested biomass divided by the total

nutrient applied annually.

The forage was analysed for macro- (N, P, K, Ca, Mg

and Na) and micronutrients (Fe, Mn, Zn and Cu). Dried,

ground samples (0Æ50 g, passed a 2-mm sieve) were dry-

ashed for 4–6 h at 500�C (Hue et al., 1997). Residues

were dissolved in 25 mL of 1 MM hydrochloric acid and

the solution subjected to inductively coupled plasma-

atomic emission spectrophotometry (Hue et al., 1997).

Ashed samples were analysed for B by the azomethine-

H colorimetric method (Wolf, 1974). Total N (0Æ25 g

sample digested at 410�C) was analysed using a LECO

CN-2000� analyzer (LECO Corporation, St. Joseph, MI,

USA). Nitrate-N and NH4+-N were auto-analysed using

spectrophotometers (Gentry and Willis, 1988).

Forage quality analysis

Crude protein (CP) content was calculated from Kjel-

dahl N using the standard conversion: CP (g kg)1) =

N(g kg)1) · 6Æ25. Acid detergent fibre (ADF) and neu-

tral detergent fibre (NDF) were determined using the

ANKOM Filter Bag Technique (ANKOM Technology

Corp., Fairport, NY, USA) using 0Æ60 g of dried, ground

tissue sample, extracted and re-dried at 102�C (2–4 h).

Calculations were based on the following formula:

NDF or ADF ðg kg�1Þ ¼ W2 � ðW1 � C1ÞSample weight

ð1Þ

where W1 is the original bag weight (g), W2 is the dried

weight of the bag with fibre after the extraction process,

and C1 is the empty bag correction, which is the empty

bag final oven dry weight divided by its original bag

weight.

Data analysis

Data collection and analysis were carried out for 2 years

(August 2004–2006). The emphasis on temporal mea-

surement and change over time enabled testing of

whether forage production, quality and soil properties

could be maintained over time with effluent irrigation.

The main treatments and interaction effects (Federer,

2005) on annual dry matter, nutrient concentration,

nutrient uptake and forage quality data were analysed

as unbalanced ANOVAANOVA using PROC GLM of SAS 9.1

software (SAS Inst., 2004). For the monthly data, best

subset procedures were conducted using MINITAB

14.13 (Minitab Inc., 2004), and the main treatments,

interaction effects and the change in yield and nutrient

uptake and removal (reduced model) were analysed

with repeated measures ANOVAANOVA using an autoregressive

first-order covariance structure provided by PROC

MIXED SAS 9.1 software (SAS Inst., 2004). Means

were compared using the least significant difference

using PROC GLM software (SAS Inst., 2004). To

facilitate data analysis and discussion, the ‘time’ vari-

able was used to refer to sampling dates that corre-

sponded to the amount of effluent irrigation over time

at each of the two irrigation rates.

Results

Plant growth and dry-matter production

Tropical grasses that received effluent irrigation exhib-

ited relatively slow growth rates during the first

15–20 d after cutting, after which these species grew

rapidly (Figure 3). The initial 27–29 d cutting resulted

in high dry-matter production and generally good-

quality forage (Figures 4 and 5); however, grasses had

slow rejuvenation after 6 months of cutting at 27- to

29-d interval. Nonetheless, tropical grasses irrigated

with effluent produced large amounts of dry matter.

Brachiaria mutica receiving the 2Æ0 ETp effluent irriga-

tion rate outyielded other grass species (P < 0Æ01), with

dry-matter production reaching about 57 Mg ha)1

year)1 (Figure 4). Significant interactions on dry-matter

yields were observed between grass species and time

(P < 0Æ05) and between irrigation rate and time

(P < 0Æ05). In general, higher dry-matter yields were



obtained in the June–October period (Figure 4), which

corresponded to the period of highest seasonal ETp with

generally higher temperature and solar radiation (Fig-

ure 6). Warmer conditions coupled with adequate

water supplied by effluent irrigation were highly

favourable for high growth rates.

Tissue concentration and uptake of N, P and K

Tissue N concentrations of the grasses (Table 1) were

low compared with the level adequate to support plant

growth and development (28Æ0 g kg)1) proposed for

Bahiagrass (P. notatum) by Mills and Jones (1996).

Paspalum notatum was chosen for comparison because of

lack of information on adequate level to support plant

growth and development specific for each grass species

used in this study. The low N levels in the tissues,

despite supplemental N fertilization in 2006, indicate

high N requirement of these grasses to support plant

growth especially with the relatively frequent harvest-

ing and low cutting height. Nitrogen levels in the soil

were also low (Table 2). Except for C. nlemfuensis

receiving the 0Æ5 ETp irrigation rate, the tissue P

concentrations of all the other grasses were also mostly

lower than the adequate level (4Æ0 g kg)1) suggested for

P. notatum. This despite the relatively high extractable

soil P (Table 2) suggests the low plant availability of soil

P because of possible precipitation with Ca (Valencia-

Gica et al., 2010). Tissue K concentrations of the grasses

exceeded the adequate level (18Æ0 g kg)1), with P. pur-

pureum having the highest (37Æ4–41Æ9 g kg)1) suggest-

ing luxury uptake of K, especially with the increased

levels of soil K with effluent application (Table 2).

Among the grasses, P. purpureum and B. mutica had

usually the highest (P < 0Æ01) N, P and K uptake

especially with the 2Æ0 ETp irrigation rate (Table 1).

Figure 3 Growth progression of tropical grasses drip-irrigated with dairy effluent, Waianae, O’ahu, Hawaii, August–October 2005.

Figure 4 Dry-matter production of tropical grasses drip-irrigated with dairy effluent, Waianae, O’ahu, Hawaii, August 2004–2006.



Figure 6 Temperature, potential evapotranspiration and solar radiation, Waianae, O’ahu, Hawaii, October 2004–August 2006.

Figure 5 Forage quality indicators of tropical grasses drip-irrigated with dairy effluent, Waianae, O’ahu, Hawaii, August 2004–2006.



Tab

le1

Ave

rage

nutr

ient

conce

ntr

atio

ns

and

annual

tota

lnutr

ient

upta

keoffo

ur

tropic

algr

asse

sre

ceiv

ing

dai

ryef

fluen

tat

Wai

anae

,O’a

hu,H

awai

i,fr

om

Augu

st2004

toA

ugu

st

2006.

Irri

gati

on

rate

Gra

sssp

ecie

s

Nu

trie

nt

co

ncen

trati

on

N

(gk

g)

1)

P

(gk

g)

1)

K

(gk

g)

1)

Ca

(gk

g)

1)

Mg

(gk

g)

1)

Fe

(mg

kg

)1)

Mn

(mg

kg

)1)

Zn

(mg

kg

)1)

Cu

(mg

kg

)1)

B

(mg

kg

)1)

2Æ0

ET

p†

Ban

agra

ss(P

enn

iset

um

pu

rpu

reu

m)

21Æ2

3Æ0

41Æ9

3Æ2

3Æ0

416

56

68

13

20

Cali

forn

ia(B

rach

iari

am

uti

ca)

24Æ8

2Æ9

35Æ2

2Æ0

2Æ3

207

81

66

11

15

Sta

rgra

ss(C

ynod

onn

lem

fuen

sis)

21Æ0

3Æ2

25Æ4

3Æ0

2Æ7

405

105

53

911

Su

ert

e(P

asp

alu

matr

atu

m)

15Æ1

2Æ0

26Æ4

3Æ9

6Æ1

1244

107

88

11

32

0Æ5

ET

p‡

Ban

agra

ss(P

.pu

rpu

reu

m)

19Æ7

3Æ4

37Æ4

3Æ6

3Æ0

526

51

56

10

15

Cali

forn

ia(B

.m

uti

ca)

18Æ5

2Æ7

30Æ4

2Æ4

3Æ0

266

64

62

10

12

Sta

rgra

ss(C

.n

lem

fuen

sis)

18Æ6

4Æ0

25Æ2

3Æ9

2Æ8

387

66

56

11

12

Su

ert

e(P

.atr

atu

m)

15Æ2

2Æ4

24Æ9

4Æ9

7Æ2

1108

98

96

10

25

LSD

0Æ0

50Æ9

0Æ2

1Æ9

0Æ3

0Æ5

240

10

14

13

Adequ

ate

level

for

Bah

iagra

ss(P

asp

alu

mn

otatu

m)

28Æ0

4Æ0

18

5Æ2

3Æ2

100

105

31

11

9

Nu

trie

nt

up

tak

e(k

gh

a)

1y

ear)

1)

2Æ0

ET

p†

Ban

agra

ss(P

.pu

rpu

reu

m)

1083

154

2141

165

155

21

2Æ8

3Æ5

0Æ7

1Æ0

Cali

forn

ia(B

.m

uti

ca)

1405

164

1992

112

130

12

4Æ6

3Æ7

0Æ6

0Æ9

Sta

rgra

ss(C

ynod

onn

lem

fuen

sis)

832

128

1007

118

108

16

4Æ2

2Æ1

0Æ4

0Æ4

Su

ert

e(P

asp

alu

matr

atu

m)

573

78

1008

154

236

49

4Æ2

3Æ3

0Æ4

1Æ2

0Æ5

ET

p‡

Ban

agra

ss(P

.pu

rpu

reu

m)

1041

179

1979

190

158

28

2Æ7

2Æ9

0Æ6

0Æ8

Cali

forn

ia(B

.m

uti

ca)

795

117

1304

101

128

11

2Æ8

2Æ7

0Æ4

0Æ5

Sta

rgra

ss(C

.n

lem

fuen

sis)

650

140

880

136

98

14

2Æ3

2Æ0

0Æ4

0Æ4

Su

ert

e(P

.atr

atu

m)

601

95

987

192

283

44

3Æ9

3Æ8

0Æ4

1Æ0

LSD

0Æ0

515

214

23

<1

<1

<1

<0Æ1

<0Æ1

†Su

mm

er

rate

was

23

mm

d)

1,

an

dw

inte

rra

tew

as

16

mm

d)

1.

‡Su

mm

er

rate

was

6m

md

)1,

an

dw

inte

rra

tew

as

5m

md

)1.

ET

p,

evapotr

an

spir

ati

on

;LSD

,le

ast

sign

ifica

nt

dif

fere

nce

.



These rates were much higher than the nutrient uptake

of P. purpureum reported in Andean Hillsides (Zhiping

et al., 2004) and Kenya (Odongo, 2002).

The N, P and K uptake of P. atratum and C. nlemfuensis

was generally lower (P < 0Æ05) compared with P. pur-

pureum and B. mutica (Table 1). Although P. atratum

appeared to grow better at the 2Æ0 ETp irrigation rate,

qualitative observation showed that it is sensitive to

prolonged effluent-saturated soil conditions. This obser-

vation contrasts with the observation that P. atratum

was well adapted to wet, lowland areas in Thailand

(Phoatong and Phaikaew, 2001).

Tissue concentration and uptake of othernutrients

Adequate levels of other nutrients aside from N, P and K

for P. notatum forage were as follows: Ca, 5Æ2 g kg)1;

Mg, 3Æ2 g kg)1; Fe, 100 mg kg)1; Mn, 105 mg kg)1; Zn,

31 mg kg)1; Cu, 11 mg kg)1; and B, 9 mg kg)1 (Mills

and Jones, 1996). Based on these levels, tissue concen-

trations in all the grasses were below adequate for Ca;

mostly adequate for Mg and Cu; above adequate for Fe,

B and Zn; and deficient to adequate for Mn (Table 1).

Sufficient to high levels of micronutrients were

observed after the supplementary foliar micronutrient

application. Overall, uptake of Fe ranged from 11Æ0 to

49Æ0 kg ha)1 year)1; Mn, 2Æ3 to 4Æ6 kg ha)1 year)1; Zn,

2Æ0 to 3Æ8 kg ha)1 year)1; Cu, 0Æ4 to 0Æ7 kg ha)1 year)1;

and B, 0Æ4 to 1Æ2 kg ha)1 year)1.

The main effects of grass species and irrigation rate as

well as their interactions on uptake of other nutrients

were significant (P < 0Æ05). Among the grasses, P. atra-

tum had the highest (P < 0Æ01) uptake of Mg at 236 and

283 kg ha)1 year)1 for the 2Æ0 ETp and 0Æ5 ETp irrigation

rates, respectively, while P. purpureum and P. atratum

receiving 0Æ5 ETp irrigation rate had the highest

(P < 0Æ01) Ca uptake of 190 and 192 kg ha)1 year)1,

respectively (Table 1). Paspalum atratum had an average

annual Mg concentration in the dry matter of 6Æ1 g kg)1

for the 2Æ0 ETp irrigation rate and 7Æ2 g kg)1 for the 0Æ5ETp rate, and Ca concentration of 3Æ9 and 4Æ9 g kg)1

respectively.

Nutrient removal as per cent of appliednutrients

The tropical grasses differed in their ability to remove

nutrients (P < 0Æ01) expressed as per cent of effluent

applied nutrient (i.e. total annual nutrient uptake in

removed biomass divided by cumulative annual nutri-

ent applied). Grasses irrigated at 0Æ5 ETp removed a

Table 2 Changes in properties of soil planted to tropical grasses receiving dairy effluent at Waianae, O’ahu, Hawaii, from August

2004 to 2006.

Irrigation

rate ⁄ sampling

depth Year pH N (%)

P

(mg kg)1)

K

(mg kg)1)

Ca

(mg kg)1)

Mg

(mg kg)1)

2Æ0 ETp†

0–15 cm 2003 7Æ50 0Æ1153 155 2229 5774 3109

2004 8Æ38 0Æ0397 164 4199 5584 3051

2005 8Æ35 0Æ0532 151 5443 5098 2966

2006 8Æ48 0Æ0363 185 8031 4926 2963

15–30 cm 2003 7Æ63 0Æ0685 93 2972 5178 3144

2004 8Æ03 0Æ0290 148 4559 5557 2932

2005 8Æ22 0Æ0489 137 5480 5088 2890

2006 8Æ29 0Æ0302 161 7695 4530 2776

0Æ5 ETp‡

0–15 cm 2003 7Æ53 0Æ0942 131 2178 5909 2991

2004 8Æ19 0Æ0321 119 2694 5847 3025

2005 8Æ23 0Æ0501 128 3140 5591 2904

2006 8Æ34 0Æ0308 142 4462 5489 3032

15–30 cm 2003 7Æ55 0Æ0772 117 2318 6750 3386

2004 8Æ09 0Æ0270 123 3138 5898 2979

2005 8Æ22 0Æ0488 129 3426 5587 2924

2006 8Æ25 0Æ0261 142 5175 5223 3042

†Summer rate was 23 mm d)1, and winter rate was 16 mm d)1.‡Summer rate was 6 mm d)1, and winter rate was 5 mm d)1.



higher (P < 0Æ01) percentage of nutrients (54–94% of N

and 44–82% of P) than those irrigated at 2Æ0 ETp (25–

62% of N and 11–23% of P) (Table 3).

Pennisetum purpureum consistently removed the high-

est (P < 0Æ05) amounts of N (94%) and P (82%) among

the grasses irrigated at 0Æ5 ETp (Table 3). Among the

grasses that received the 2Æ0 ETp rate, B. mutica removed

the highest percentage of applied nutrient (62% N and

23% P), but these values were not significantly differ-

ent from the percentage N and P removed by the other

grasses (P > 0Æ05). Removal percentages for K (2–13%),

Ca (3–15%) and Mg (1–12%) were rather low, imply-

ing a possible accumulation or leaching of these

nutrients through the soil. This has important implica-

tions on the possible Ca-P precipitation and eventual

accumulation of these nutrients in the soil.

Forage quality

Crude protein

High crude protein (>150 g kg)1) forages are desirable

for sustained milk yield (22–32 kg milk d)1) of lactating

cattle (NRC, 2001). In this study, the average CP

contents of effluent-irrigated grasses were between 90

and 160 g kg)1 during the 2-year period (Figure 5).

Averaged across species and irrigation rate, all grasses,

except for P. atratum, had generally higher average CP

content when irrigated at 2Æ0 ETp rate (P < 0Æ01).

Neutral detergent fibre

The energy value of forage is often evaluated using

NDF, which is composed of cellulose, hemicellulose and

lignin (NRC, 2001). In this study, average forage NDF

ranged from 570 to 620 g kg)1 over the 2-year period,

with a significant (P < 0Æ01) main effect of time and an

interaction of grass species and irrigation rate. Higher

forage NDF was observed during the June–November

period when solar radiation and temperature were

usually higher (Figure 5).

Acid detergent fibre

The ADF is composed of the least digestible parts of cell

walls such as cellulose, and lignin (NRC, 2001). The

ADF of the grasses irrigated with effluent changed with

time (P < 0Æ01) and differed between grass species

(P < 0Æ01) and irrigation rate (P < 0Æ01). The average

ADF of effluent-irrigated grasses ranged from 320 to

360 g kg)1 (Figure 5). As with the NDF, the higher ADF

values (up to 400 g kg)1) were obtained during the

period when solar radiation and temperature were

higher (June–November) (Figure 5).

Discussion

The selected tropical grasses maintained their high

productivity when drip-irrigated with dairy effluent at

2Æ0 ETp rate as evidenced by the higher dry-matter

yields and nutrient uptake at this irrigation rate than at

the 0Æ5 ETp rate. The 2Æ0 ETp rate of application is

approximately double the rate typically applied for

irrigation to meet the crop water requirement. This high

rate of application indicates the high capacity of tropical

forage grasses to utilize large amounts of lagoon waste

water, if needed.

Effluent-irrigated tropical grasses produced high lev-

els of biomass that were as high or higher than values

commonly reported in tropical literature. Among the

Table 3 Nutrient removal by various tropical grasses receiving dairy effluent at Waianae, O’ahu, Hawaii, from August 2004 to

2006.

Irrigation

rate Grass species

Nutrient removal (% of annually applied nutrients)

N P K Ca Mg Fe Mn Zn Cu B

2Æ0 ETp† Banagrass (Pennisetum purpureum) 48 21 5 4 2 35 28 191 3 2

California (Brachiaria mutica) 62 23 4 3 2 19 45 206 3 1

Stargrass (Cynodon nlemfuensis) 37 18 2 3 1 26 41 115 2 1

Suerte (Paspalum atratum) 25 11 2 4 3 81 41 180 2 2

0Æ5 ETp‡ Banagrass (P. purpureum) 94 82 13 15 7 159 83 536 9 6

California (B. mutica) 72 54 9 8 6 65 86 489 7 3

Stargrass (C. nlemfuensis) 59 65 6 10 4 77 72 359 6 3

Suerte (P. atratum) 54 44 7 15 12 250 121 694 6 6

LSD0Æ05 30 11 2 8 4 201 105 383 5 1

†Summer rate was 23 mm d)1, and winter rate was 16 mm d)1.‡Summer rate was 6 mm d)1, and winter rate was 5 mm d)1.

ETp, evapotranspiration; LSD, least significant difference.



grasses, B. mutica and P. purpureum produced the high-

est dry-matter yields and demonstrated the greatest

potential to absorb nutrients from the effluent. In

support of the observed high forage productivity, the

calculated carrying capacity was 8–11 lactating cows

ha)1 year)1 using the dry-matter yields of B. mutica and

P. purpureum and milk production of 9 kg d)1 of 4% fat-

corrected milk (NRC, 2001) for a 607-kg cow (Wilker-

son et al., 1997) with dry-matter intake of 21 g kg)1 of

body weight (NRC, 2001).

The predicted nutrient uptake from intensively man-

aged, irrigated tropical pasture grasses approached 750–

1500 kg N ha)1 year)1 and 75–150 kg P ha)1 year)1

(Russell S. Yost, personal communication). The N and

P uptake of effluent-irrigated grasses was mostly within

this predicted range of nutrient removal. Despite the

deficient P level in forage tissues, P concentrations (2Æ0–

4Æ0 g kg)1) were all within the typical range of tissue P

concentrations (1Æ5–4Æ0 g kg)1) reported for grasses

grown in heavily manured soils or certain other high

P soils with relatively low P sorption capacities (Math-

ews et al., 2005). The P content of the forage of effluent-

irrigated grass species in this study meets the dietary P

requirement of dairy cows according to NRC (2001)

guidelines. Farmers feeding their cattle with forage

from these species may reduce or eliminate the

supplementation of the ration with imported P supple-

ments for sustained milk production.

The recommended Mg concentration in a dairy cattle

ration is from 2Æ5 to 3Æ5 g kg)1 and slightly higher for

early lactating, highly productive cows (4Æ5 g kg)1)

(Harris et al., 1994), but the Mg level in the forage

should not be lower than 2Æ0 g kg)1 to prevent grass

tetany (Voisin, 1963). The high Mg content of P. atra-

tum forage is, therefore, favourable for avoiding the

occurrence of grass tetany in dairy cattle. Farmers

should be cautioned, however, that very high levels of

Mg may favour hypocalcaemia in dairy cows (Chester-

Jones et al., 1990), especially if plant Ca levels are low

as the case in this study. In contrast, the high levels of K

in plant tissues as in the case of P. purpureum reduce the

availability of plant Mg in animals (Fisher et al., 1994).

Potassium is very important in dairy cattle diet, being

a major mineral component of milk. Generally, cattle

require low amounts of K, a minimum of 1Æ0% or if

under heat stress, up to 1Æ5%, of the total ration dry

matter (NRC, 2001). Although the K concentration in

the dry matter of P. purpureum was relatively high

(Table 1), it can be balanced with other components of

the ration to achieve the acceptable K concentration.

Some studies have implicated the high level of K in

forage to milk fever or parturient paresis, an abnormal

physiological event in lactating cattle characterized by

rapid reduction in plasma Ca concentrations (Goff and

Horst, 1997).

Results also support other researchers’ findings (Cla-

vero, 1997; Wijitphan et al., 2009) that harvesting

interval and cutting height affect grass growth and

productivity. For example, effluent-irrigated grasses

exhibited slower rejuvenation when cut at 27- to 29-d

interval. The slow recovery after cutting was probably

exacerbated by the low cutting height (8 cm) and the

pressure from the relatively heavy weight of the

harvesting equipment. This reduction in growth and

productivity was greatest on P. purpureum and P. atra-

tum. The growth habit of P. atratum is normally char-

acterized by clump-producing culms. Pennisetum

purpureum also has tussocky growth with branching

upper culms. With low cutting height, these grasses

turned to a prostrate growth habit with higher produc-

tion of basal tillers. In contrast, B. mutica and C. nlemf-

uensis have spreading growth habit with rooting

runners. Thus, the low cutting height did not adversely

affect their growth and productivity.

During the first 3 months of the experiment when cut

at 30 cm height, dry-matter yield of P. purpureum, for

example, was 9 t ha)1 month)1 (Figure 3). This yield

went down to a low 2 t ha)1 month)1 when cutting

height was reduced to 8 cm. At low cutting height

(8 cm), P. purpureum and P. atratum initially produced

more tillers, but after 6 months, these grasses started to

exhibit slower growth and less tiller production. The low

cutting height may have resulted in the removal of

growing points, particularly the apical meristems of the

tillers, preventing them from producing new leaves. This

may have resulted in reduced carbohydrate production

and storage in the stems of tillers and eventually tiller

death (Clavero, 1997; Wijitphan et al., 2009). During the

second year when harvesting interval was increased

(42–44 d), grasses showed the ability to recover better

despite the low cutting height.

Deficiencies typical of micronutrients were observed

as a general chlorosis of new growth after the fourth

month of effluent irrigation. This was likely a conse-

quence of the high soil pH, which resulted in low

micronutrient availability (Moraghan and Mascagni,

1991). In addition, dairy effluent supplied only minimal

quantities of micronutrients (data not shown). In

addition, the relatively high extractable P in the soil

may have resulted in antagonistic interactions with Zn

(Alloway, 2008). With supplemental micronutrient

fertilization, chlorosis had become minimal and in-

creases in the micronutrient levels in the forage were

observed. Supplemental applications of micronutrients

(Zn, Cu, Fe and Mn) and N were therefore necessary to

maintain acceptable tissue levels of these nutrients and

prevent chlorosis.

Effluent-irrigated grasses had CP contents that were

mostly below 150 g kg)1 possibly due to relatively

longer cutting intervals (28–52 d). High forage CP levels



(>150 g kg)1) are usually obtained on forages that are

well fertilized with N and cut early (17–21 d) (Chin,

1995). In terms of NDF, the values were higher than the

recommended minimum values of the forage intended

for lactating cows (NRC, 2001), but mostly lower than

the range (600–750 g kg)1) reported for many cool-

season and tropical grasses (Nelson and Moser, 1994).

For example, the NDF of these grasses was lower than

that reported by Ruiz et al. (1995) for silages made from

P. purpureum (680 g kg)1) and C. dactylon (745 g kg)1).

However, Sarwar et al. (1992) found that increasing

NDF from 320 to 480 g kg)1 did not reduce the dry-

matter intake and that a minimum of 600 g kg)1 NDF

had little effect on milk production.

Both NDF and ADF of the grasses used in this study

were generally higher in the June–November period.

Warmer temperatures reportedly enhance lignin syn-

thesis resulting in lower digestibility (Buxton and Fales,

1994; Nelson and Moser, 1994). Paspalum atratum,

C. nlemfuensis and B. mutica all flower in October–

November, which could explain the somewhat higher

NDF observed at that time. Forage nutritive value

decreases with increasing plant maturity because of

increasing structural carbohydrate concentration (ADF,

NDF and lignin), with the greatest increase occurring

between booting and anthesis (Holman et al., 2007).

Based on NDF and ADF values measured from effluent-

irrigated grass species, an adjustment in the dietary

formulation with allowance for higher proportions from

concentrates may be needed when feeding lactating

cows with tropical grasses to meet the NRC require-

ments for NDF and ADF. The recommended minimum

dietary NDF values are 250–290 g kg)1 during early

lactation and 320–340 g kg)1 during mid- to late

lactation (NRC, 2001) to allow cattle to eat more

forage. For forage ADF, the recommended minimum is

170–210 g kg)1, but as with NDF, a higher minimum is

required for forage ADF depending on various factors

that also affect NDF such as particle size, feeding

methods, supplements, and rate and extent of ferment-

ability of fibre source, among others (NRC, 2001).

Based on this information, forage quality in this study

such as CP, NDF and ADF was at levels that are

acceptable for feeding lactating cattle. However, for

high-yielding cows, as NDF percentage increases, dry-

matter intake generally decreases (Varga et al., 1998).

Although a minimum NDF value is needed to keep

proper rumen and digestion conditions, generally high

values of NDF are not adequate to maintain high milk

production levels, especially for high-yielding lactating

cows, because of the effect on intake. Because NDF is

negatively related to forage digestibility and positively

related to fill, daily intake may also be reduced for

forages with low NDF values (Varga et al., 1998). As

with NDF, higher forage ADF results in reduced

digestible dry matter as a consequence of increased

lignification of cellulose in the plant (DePeters, 1993).

Forage with higher ADF, thus, has lower cellulose

digestibility in the rumen, thereby reducing the energy

available to the lactating cow for milk production.

In conclusion, B. mutica and P. purpureum appeared to

be excellent choices for forage production to maximize

reuse of dairy effluent. Effluent irrigation for forage

production appears to be an effective means of recycling

energy-expensive nutrients, while reducing the build-

up of effluent in lagoons and, in turn, reducing the risk

of overflow and contamination of associated water

bodies. This recycling partially closes the open nutrient

cycle in the dairy production system. With the 2Æ0 ETp

treatments, however, there is a possibility of overwa-

tering and excess nutrient application that could lead to

build-up of nutrients in the soil or leaching of nutrient

to the environment. This study is preliminary and

assessing such impacts is beyond the scope of this

manuscript. Further studies with high application rates

of animal effluent beyond two years should be con-

ducted to monitor soil nutrient levels and their

potential environmental impacts.

Acknowledgments

The authors thank the USDA T-Star Program of the

University of Hawaii at Manoa, Honolulu, Hawaii,

USA, for the project funding and research assistant-

ship support of the lead author. We also thank the

CSREES ⁄ USDA 406 Water Quality Initiative Compet-

itive Grants Program, Honolulu, Hawaii, USA, for

partial funding. We also acknowledge Veronica

Wilcox for the initial grass planting and establish-

ment, Zhijun Zhou for the design and establishment

of the irrigation system, and colleagues in the

laboratory for their invaluable help in soil sample

collection and grass harvesting. Reference herein to

any specific product, by trade name, trademark,

manufacturer or distributor, does not necessarily

constitute or imply its endorsement or recommenda-

tion by the authors and publishers.

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