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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
� 2012 Blackwell Publishing Ltd. Grass and Forage Science
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
4 R. B. Valencia-Gica et al.
� 2012 Blackwell Publishing Ltd. Grass and Forage Science
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.
Productivity of effluent-irrigated grasses 5
� 2012 Blackwell Publishing Ltd. Grass and Forage Science
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.
6 R. B. Valencia-Gica et al.
� 2012 Blackwell Publishing Ltd. Grass and Forage Science
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
.
Productivity of effluent-irrigated grasses 7
� 2012 Blackwell Publishing Ltd. Grass and Forage Science
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.
8 R. B. Valencia-Gica et al.
� 2012 Blackwell Publishing Ltd. Grass and Forage Science
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.
Productivity of effluent-irrigated grasses 9
� 2012 Blackwell Publishing Ltd. Grass and Forage Science
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
10 R. B. Valencia-Gica et al.
� 2012 Blackwell Publishing Ltd. Grass and Forage Science
(>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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