Agricultural and Food Science, Vol. 16 (2007): 17–24

Vol. 16 (2007): 17–24

A G R I C U L T U R A L A N D F O O D S C I E N C E

Feeding value of low quality grass silage
supplemented with maize silage for sheep

Marina Vranić, Mladen Knežević, Krešimir Bošnjak, Josip Leto, Goran Perćulija
Faculty of Agriculture, University of Zagreb, Department of Field Crop Production,

Grassland Management Centre, Svetošimunska cesta 25, 10000 Zagreb, Croatia,
e-mail: mpavlak@agr.hr

The objective of this experiment was to study the effects of interactions between low quality grass silage
(GS) dominated by orchardgrass and maize silage (MS) on ad libitum intake, digestibility and nitrogen
retention in wether sheep. The study consisted of four feeding treatments involving GS and MS alone and
GS and MS mixtures in a ratio of 67:33 or 33:67 (dry matter (DM) basis) fed twice daily. The GS was high
in DM (463 g kg–1), neutral detergent fibre (715 g kg–1DM) and acid detergent fibre (429 g kg–1DM) while
low in crude protein (90.1 g kg–1DM). The DM content (g kg–1) and starch concentration (g kg–1DM) of MS
were 264 and 211, respectively. The inclusion of MS into diet had positive linear effects on fresh matter ad
libitum intake (kg d–1 and g kg–1M0.75d–1) (P < 0.01 and P < 0.001 respectively), digestibility of DM (P <
0.01), organic matter (P < 0.01), acid detergent fibre (P < 0.05), starch (P < 0.001), digestibility of organic
matter in DM (D-value) (P < 0.001), nitrogen intake (P < 0.01) and nitrogen output in faeces (P < 0.01).
A positive associative effect of low quality GS and MS was observed for ad libitum intake (kg d–1 and g
kg–1M0.75d–1) of fresh matter (quadratic, P < 0.01), DM (quadratic, P < 0.001 and P < 0.01 respectively)
and organic matter (P < 0.001), for digestibility of DM, neutral detergent fibre, acid detergent fibre, crude
protein, starch and D-value (quadratic, P < 0.01), digestibility of organic matter (quadratic, P < 0.05), nitrogen
intake (quadratic, P < 0.001) and nitrogen balance (quadratic, P < 0.05). It was concluded that differences
between low quality GS and MS resulted in positive associative responses of GS and MS for all parameters
measured (intake, digestibility and nitrogen retention).

Key-words: grass silage, maize silage, intake, digestibility, nitrogen retention

Introduction

Many sheep producers in Croatia utilize conserved
forages such as grass silage (GS) in their winter-
feeding program. However, GS produced at family

farms is often of low nutritive value due to its high
concentration of fibre, low digestibility and low
concentration of crude protein (CP) (Vranić et al.
2005a). Improvement of digestibility and intake
are the two major factors for raising the nutritive

Vranić, M. et al. Feeding value of low quality grass silage for sheep

A G R I C U L T U R A L A N D F O O D S C I E N C E
Vol. 1 (2007): 17–24

value of low quality forage for ruminants. One
way of improving utilization of low quality GS
is to increase microbial activity in the rumen by
supplementing the diet with feeds high in rumen
degradable organic matter and thereby increase
microbial protein synthesis and short chain fatty
acids production. Maize silage (MS) may be used as
supplemental forage to GS because it complements
grass silage well. Previous investigations with
sheep have shown increased intake and digestibility
when GS was partially replaced with a supplemen-
tal energy source (Rouzbehan et al. 1996). Margan
et al. (1994) observed positive associative effects
of MS and red clover hay for voluntary intake,
digestibility of nitrogen (N), organic matter (OM)
and N balance.

When fed in combination, associative effects
depend on the quality of GS and are also related to
the maturity of MS (Hameleers 1998). Positive re-
sponses could be expected when the GS to be re-
placed was of lower quality than the included for-
age substitute (Weller et al. 1991). As sheep prefer
maize to grass silage diet (O’Doherty et al. 1997)
and the GS in this study was of low quality, the
combination of GS and MS was offered under the
hypothesis that feeding a mixture of these supple-
ments would have positive associative effects on
food intake, digestibility and N retention in sheep.
The objective of this experiment was to examine the
effects of interactions between the low quality GS
dominated by orchardgrass and MS on feed intake,
digestibility and N retention in wether sheep.

Material and methods
Sward and silage making

The GS was made from a semi-permanent, pre-
dominantly orchardgrass (Dactylis glomerata
L.) meadow harvested on 6 June 2002, primary
growth, late bloom stage. During the growing
season two applications of a commercial inorganic
fertilizer were provided. In February 2002, 450 kg
ha–1 N-P-K fertilizer (8:26:26), and thirty-five days
prior to harvesting 150 kg ha–1 of ammonium nitrate
were applied.

Green and dry matter (DM) yield (t ha–1) was
determined at mowing by calculating the weight
of 30 forage samples randomly taken by a quad-
ratic frame (0.25 × 0.25 m). Botanical composition
was determined from the same samples by manu-
al separation of sward components (grasses, clo-
vers, forbs).

The sward contained 80.6% orchardgrass (Dac-
tylis glomerata L.), 13.7% legumes (11.2% white
clover and 2.5% red clover), 2.3% other grasses and
3.4% forbs on a DM basis. Forage DM content at
harvest was 276 g kg–1 fresh sample and DM yield
was 7.01 t ha–1. The crop was allowed to wilt for
24 h before harvesting with a round baler. Bales
were wrapped in 4 layers of 500 mm-wide white
plastic film. The weather at harvest was warm and
sunny. No additive was applied.

Forage maize crop (Zea mays L., cultivar BC
566) was sown on 8 March 2002 into a prepared
(ploughed and rolled) seedbed. The crop was sown
with a row space of 75 cm and the establishment
target was 70000 plants ha–1. Whole crop maize
was harvested on 23 September 2002 to a nominal
stubble height of 25 cm above ground (pre-harvest
DM of 275 g kg–1 fresh weight). The DM yield of
forage maize at harvest was 13.5 t ha–1, while the
cob DM to total DM ratio was 6:1. The forage was
chopped at harvest to standard chop length, ensiled
into a clamp silo immediately, without any addi-
tive, and rolled thoroughly before being sheeted
with plastic and covered with rubber tyres to en-
sure exclusion of air.

Dietary treatments
The treatments consisted of either GS or MS
alone, or a forage mixture (DM based) of GS and
MS of 670 g kg–1 GS and 330 g kg–1 MS (GGM)
or 330 g kg–1 GS and 670 g kg–1 MS (MMG). Just
before the experiment started, the MS for experi-
mental needs was compressed into 8 plastic contain-
ers (approximately 200 l each) and stored in a cold
chamber maintained at a temperature of 4ºC.

The GS was chopped to approximately 3–5 cm
using a commercial chopper. The chopped material
was compressed into plastic bags (approximately

Vranić, M. et al. Feeding value of low quality grass silage for sheep Vol. 16 (2007): 17–24

A G R I C U L T U R A L A N D F O O D S C I E N C E

20 kg GS per bag) under continuous CO2 flushing
and stored in a cold chamber (4ºC). Prior to feed-
ing, the forage was mixed weekly and held in plas-
tic bags in a cold room (4°C) to prevent heating.
No supplementary feeds were provided.

Animals and experimental design
Ten Charolais wethers were selected on the basis
of their live weight (mean body weight 43.5 kg,
s.d. 3.8 kg) and condition score. All animals were
treated for internal parasites prior to the start of
experiment. The sheep were subjected to artificial
lightening from 0800 to 2000 daily. Each sheep
was randomly allocated to treatment sequences in
an incomplete changeover design with four periods.
A 10-day acclimatization period was followed by
an 11-day measurement period (4-day ad libitum
intake was followed by 7-day digestibility and
N retention measurements) where feed offer and
refusals were measured and total urine and faeces
were collected.

The animals were housed in individual pens (1.5
× 2.2 m) over the acclimatization period and in indi-
vidual crates (136 cm × 53 cm × 148.5 cm) during
the measurement period. Diets were offered twice a
day (0830 and 1600) in equal amounts, designed to
ensure a refusal margin of 10–15% each day. Dur-
ing the measurement period, the fresh weights and
DM contents of feed offered and feed refused were
recorded daily. Subsamples of the feed “as offered”
were taken daily and stored at –20ºC until the end
of the experiment, when they were bulked prior
to chemical analysis. Daily subsamples of refus-
als were bulked on an individual animal basis and
stored at –20ºC prior to chemical analysis.

Daily production of urine and faeces were col-
lected separately. Daily output of urine from each
animal was preserved by acidification (100 ml of
2 mol l–1 sulphuric acid to achieve a pH value of
2–3) and its volume was measured. Daily subsam-
ples of urine from individual animals were then
bulked across the measurement week and stored
at –20ºC until analysis.

Total daily faecal production of each animal was
stored frozen until completion of the collection peri-

od. The bulked faecal output from each animal was
then weighed and subsampled prior to subsequent
analysis. The sheep were weighed on the 10th, 14th
and 21st day of each period and the mean weight
was used to calculate daily voluntary intake of fresh
matter (FM), DM and OM expressed per unit of
metabolic weight, i.e., g per kg M0.75.

Chemical analysis
The DM contents of feed offered, feed refused and
faeces were determined by oven drying to a constant
weight at 60ºC in a fan-assisted oven (ELE Interna-
tional). Ash was measured by igniting samples in
a muffle furnace (Nabertherm) at 550ºC for 16 h.
Total N concentrations of feed offered, feed refused,
faeces and urine were determined by the Kjeldahl
method (AOAC 1990, ID 954.01) using a Gerhardt
nitrogen analyzer. Additionally, N concentration
was expressed as CP (total N × 6.25) g kg–1 DM for
feed offered, feed refused and faeces.

Acid detergent fibre (ADF) and neutral deter-
gent fibre (NDF) were measured using the proce-
dure of Van Soest et al. (1991). Silage pH was de-
termined in a water extract from 10 g of fresh si-
lage and 100 ml distilled water using the pH me-
ter 315i (WTW). Starch content of the feed of-
fered, feed refused and faeces was determined by
polarimetry (Ministry of Agriculture, Fishers and
Food 1982).

Statistical analysis
Results were analyzed using mixed model proce-
dures (SAS 1999). Mean separation of chemical
composition of grass and maize silage and their
mixtures was calculated using the LSD values if
the F-test was significant at P = 0.05. Linear and
quadratic effects of the level of MS inclusion in GS
on ad libitum intake, digestibility and N utilization
were examined using the CONTRAST statement of
SAS. Model applied: Yij = µ + Ti + Pj + eij, where Y
is the overall model, µ = grand mean, T = treatment,
P = period, e = experimental error, I = number of
treatments, and j = number of periods.

Vranić, M. et al. Feeding value of low quality grass silage for sheep

A G R I C U L T U R A L A N D F O O D S C I E N C E
Vol. 1 (2007): 17–24

tion (P < 0.001). Lactic acid was the major organic
fermentation acid in the silages and pH ranged from
3.7 to 4.6.

Intake and digestibility
Table 2 shows FM, DM and OM ad libitum intake
and total tract apparent digestibility of GS, MS
and their mixtures fed to wether sheep. Silage FM
intake (kg d–1 and g kg–1M0.75d–1) increased linearly
(P < 0.01 and P < 0.001 respectively) as the propor-
tion of MS in the diet increased. Diet FM intake
(kg d–1 and g kg–1 M0.75 d–1) responded quadratically
(P < 0.01) to increasing levels of MS and so did diet
DM (P < 0.001 and P < 0.01 respectively) and OM
intake (P < 0.001).

Addition of MS linearly increased apparent di-
gestibility of DM (P < 0.01), OM (P < 0.01), ADF (P
< 0.05), starch (P < 0.001) and digestibility of OM in
DM (D-value) (P < 0.001). Digestibility of DM, NDF,
ADF, CP, starch and D-value responded quadratically
(P < 0.01) and so did OM digestibility (P < 0.05) as
the proportion of MS increased in the diet.

Results

Diet chemical composition
The chemical composition of GS, MS and mixtures
of the two forages is presented in Table 1.

Inclusion of MS into GS (33 vs. 67%) reduced
the DM content of the diet (P < 0.001) due to much
lower DM in MS than GS (P < 0.001). Maize si-
lage was lower in CP than GS (P < 0.001), which
progressively lowered the CP concentration in the
diet with both levels of MS inclusion (33 vs. 67%)
(P < 0.001). Forage mixture of 330 g kg–1 GS and
670 g kg–1 MS had a lower CP concentration (P <
0.001) compared to GGM.

Grass silage contained less OM than MS (P <
0.001) but a higher concentration of NDF (P < 0.05)
and ADF (P < 0.001). In contrast, MS contained
more non-structural carbohydrates, such as starch
(P < 0.001), than GS. Therefore, with increasing the
MS inclusion in forage mixtures, a reduction was
expected in NDF (P < 0.05) and ADF (P < 0.001)
concentration and an increase in starch concentra-

Table 1. Chemical composition of grass and maize silage and their mixtures (g kg–1DM, unless otherwise stated).

Grass
silage GGM MMG

Maize
silage SED Significance

Dry matter (DM) (g kg–1 fresh weight) 463a 412b 345c 264d 0.78 ***
DM composition (g kg–1DM)
Organic matter 914d 923c 933b 955a 1 ***
Crude protein 90.1a 84.7b 79.7c 62.0d 1.2 ***
Neutral detergent fibre 715a 694a 674b 582c 13 *
Acid detergent fibre 429a 407b 374c 321d 6 ***
Starch 14.7d 46.7c 96.5b 211a 7.7 ***
Fermentation characteristics (g kg–1 DM)
Lactic acid 78.7 84.5 92.4 93.7 ND
Acetic acid 36.9 43.2 49.8 67.1 ND
Butyric acid NF NF NF NF ND
Ammonium nitrogen, g kg–1 total N 128.6 132.0 145.0 165.2 ND
pH 4.6a 4.2b 4.1b 3.7c 0.08 ***

GGM = grass silage 670 g kg–1 DM, maize silage 330 g kg–1 DM, MMG = maize silage 670 g kg–1 DM, grass silage 330
g kg–1 DM. NF = not found, ND = not determined, SED = standard error of difference
Values within the same row with different superscripts differ significantly (*, P < 0.05; ***, P < 0.001).

Vranić, M. et al. Feeding value of low quality grass silage for sheep Vol. 16 (2007): 17–24

A G R I C U L T U R A L A N D F O O D S C I E N C E

Nitrogen balance

Table 3 shows N utilization of GS, MS and their
mixtures. Nitrogen intake and N output in faeces
were linearly affected (P < 0.01) by the MS inclu-
sion in the diet. Nitrogen intake responded quadrati-
cally (P < 0.001) to increasing levels of MS and so
did N balance (P < 0.05). Negative N balance was
found in sheep fed MS only.

Discussion
The average CP content of GS used in this experi-
ment was between 77 and 167.5 g kg–1 DM deter-
mined as minimum and maximum average values
for grass silages produced at 19 family farms in
Croatia in 2004 (Vranić et al. 2005a). Relatively
high DM content of GS was a result of advanced grass
maturity and 24-hour wilting prior to harvest.

Table 2. Fresh matter, dry matter, organic matter ad libitum intake and total tract digestibility of grass silage, maize
silage and their mixtures fed to wether sheep.

Grass
silage

GGM MMG Maize
silage

SEM Significance of

L Q

Voluntary intake
Fresh matter (kg d–1) 2.36 3.52 4.06 3.62 0.22 ** **
Dry matter (kg d–1) 1.08 1.45 1.42 0.93 0.08 NS ***
Organic matter (kg d–1) 0.99 1.34 1.32 0.87 0.07 NS ***
Fresh matter (g kg–1 M0.75 d–1) 129 189 216 206 8.89 *** **
Dry matter (g kg–1 M0.75 d–1) 59.0 80.7 79.2 49.6 5.65 NS **
Organic matter (g kg–1 M0.75 d–1) 54.5 71.9 70.6 49.8 3.57 NS ***

Digestibility (g kg–1)
Dry matter 487 628 669 631 24.6 ** **
Organic matter 495 644 684 651 31.3 ** *
Neutral detergent fibre 514 650 667 595 30.9 NS **
Acid detergent fibre 454 604 630 562 32.1 * **
Crude protein 489 570 568 469 30.6 NS **
Starch 948 990 995 998 4.5 *** **
D-value (g kg–1 DM) 476 594 637 617 19.4 *** **

GGM = grass silage 670 g kg–1 DM, maize silage 330 g kg–1 DM, MMG = maize silage 670 g kg–1 DM, grass silage
330 g kg–1 DM. SEM = standard error of the mean. L = Linear effect of maize silage in the diet, Q = Quadratic effect of
maize silage in the diet, NS = not significant, * P < 0.05; ** P < 0.01; *** P < 0.001. D-value = digestible organic mat-
ter in the dry matter. M0.75 = metabolic body weight.

The DM content of MS used in this experi-
ment (264 g kg–1) was much lower than the average
two-year DM content of maize silages for Croatia
(372.38 g kg–1) (Vranić et al. 2005b), and when
viewed in conjunction with its medium starch con-
tent of 211 g kg–1DM is indicative of less mature
maize silage. The reason was an unusually wet sum-
mer in 2002, which prolonged the growth of maize
crop and resulted in lower DM and starch concen-
tration at harvest. Expected differences in the car-
bohydrate components of the two crops were ap-
parent, with GS containing more ADF and NDF
than MS. Lower pH for MS was probably related
to lower DM concentration and lower buffering
capacity of MS compared with GS. In restrictively
fermented GS the water soluble carbohydrates can
be at the level of the fresh grass, whereas in exten-
sively fermented silage they have been mainly ex-
hausted (Jaakkola et al. 2006).

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Vol. 1 (2007): 17–24

Voluntary DM intakes across the four feeding
treatments approached or exceeded the upper lim-
it of the intake range of 800–1100 g d–1 for 50 kg
intact male lambs (AFRC 1993). The increasing
level of MS linearly increased FM intake while a
positive associative effect of the two forages was
observed for FM, DM and OM intake. It has been
suggested that forage NDF content (Van Soest et
al. 1991) and digestibility, especially NDF digest-
ibility (Anil et al. 2000) are important in the regu-
lation of forage intake. Also, for low quality forag-
es, intake is regulated predominantly by physical
factors, principally the physical fill in the rumen.
Therefore, higher NDF content (715 g kg–1DM) and
lower NDF digestibility (514 g kg–1) of GS in com-
parison with the NDF content (582 g kg–1DM) and
NDF digestibility (595 g kg–1) of MS resulted in a
linear increase in diet FM intake with the increas-
ing level of MS and in positive associative effects
of the two forages for voluntary intake. Although
GS and MS used in this experiment were both low
in CP content, which resulted in limited N supply
to rumen microorganisms, the supplemented ener-
gy in the form of MS improved microbial activity
by developing a better environment for rumen fer-
mentation and reduced indigestible materials of the
diets (Matsui et al. 1998).

The in vivo digestibility of total diets, when de-
termined with wether sheep, quadratically increased
for all parameters with the increasing level of MS.
This was a reflection of the higher in vivo digest-
ibility obtained with MS for all parameters except

CP when silages were fed as the sole diet. Higher
CP digestibility in the GS diet than MS diet may
be due to the fact that MS has a lower CP level and
thus the impact of metabolic faecal nitrogen in caus-
ing apparent CP digestibility is lower with the MS
than the GS diet (O’Mara et al. 1998).

Digestibility of starch was much higher than
that of NDF, which is consistent with the results
of Firkins et al. (2001) that, on average, the appar-
ent digestibility of starch is almost twice as high as
that of NDF. In this experiment, starch digestibil-
ity in MS diet was high (998 g kg–1 DM) and simi-
lar to the value of 990 g kg–1 DM reported by Anil
et al. (2000) for starch digestibility in MS of sim-
ilar quality determined in wether sheep. This fur-
ther supports the linear increase in diet digestibil-
ity with the increasing level of MS, since reduced
starch digestibility accounts for approximately one-
half of depression in the MS digestibility (Joan-
ning et al. 1981).

The intake of N was affected by the energy lev-
el of the diet and the sheep fed higher energy diets
(GGM, MMG) consumed more N than the sheep
fed GS diet. The intake of N increased linearly as
the NDF:starch ratio increased, as a result of a pos-
itive associative effect of the two forages in the in-
take of forage mixtures. Higher N output in urine
and faeces (9.7 g d–1) than N input (9.46 g d–1) was
recorded in lambs offered the MS diet, which led
to negative N balance. Nitrogen output in urine
and faeces for diets containing GS was estimated
between 63 and 81.4% of N intake (GGS and GS

Table 3. Nitrogen utilization of grass silage, maize silage and their mixtures fed to wether sheep.

Grass
silage

GGM MMG Maize
silage

SEM Significance of

L Q
Nitrogen balance (g d–1)
Nitrogen intake 16.2 20.0 18.3 9.46 1.18 ** ***
Nitrogen output in faeces 8.3 8.6 7.9 5.8 0.55 ** NS
Nitrogen output in urine 4.9 4.0 5.8 3.9 1.12 NS NS
Nitrogen balance 2.9 7.5 4.5 -0.28 1.69 NS *

GGM = grass silage 670 g kg–1 DM, maize silage 330 g kg–1 DM, MMG = maize silage 670 g kg–1 DM, grass silage 330
g kg–1 DM. SEM = standard error of the mean, L = Linear effect of maize silage in the diet, Q = Quadratic effect of maize
silage in the diet. NS = not significant, * P < 0.05; ** P < 0.01; *** P < 0.001

Vranić, M. et al. Feeding value of low quality grass silage for sheep Vol. 16 (2007): 17–24

A G R I C U L T U R A L A N D F O O D S C I E N C E

diet, respectively), indicating an inefficient micro-
bial capture of rumen degradable N. This is partly
supported with the results of Fraser et al. (2000) that
nitrogen excretion in faeces and urine accounts for
a high proportion of N intake, which may be more
than 70% of the daily N consumption. Despite a
similar urinary and faecal N loss in the diets con-
taining GS, a positive associative effect of two for-
ages on N retention was recorded due to a positive
associative effect on N intake. Nitrogen retention
has been shown to be lower with the forage of lower
CP concentration due to decreased DM intake and
CP digestibility (Ko et al. 2006), which may account
for the low nitrogen balance of GS diet. Conversely,
higher N balances recorded for GGM and MMG in
comparison with GS diet were due to positive as-
sociative effects of the two forages.

Positive associative effects have been noted
when different forage sources, such as grasses and
legumes, are fed in combination (Hunt et al. 1985).
These effects are usually only observed when one
forage source supplies a nutrient, most often pro-
tein, which is deficient in the other forage source.
Positive associative responses in intake and di-
gestibility are commonly noted when protein sup-
plements are provided to ruminants fed low qual-
ity forage (Hannah et al. 1991). Thus, Margan et
al. (1994) reported positive associative effects for
voluntary intake, digestibility of N, OM and N bal-
ance of MS and red clover hay that contained as
much as 231 g CP kg–1 DM. Both GS and MS used
in this experiment were low in CP content, but MS
was much higher in starch as an important source
of energy for ruminants. These differences result-
ed in positive associative responses of GS and MS
for all parameters measured (intake, digestibility
and nitrogen balance).

The results of this experiment might be useful
to producers who find it difficult to consistently
produce maize silage with DM contents close to or
above 300 g kg–1, since they indicate that moderate
quality maize silage has a potential to increase diet
quality when it replaces low quality grass silage.

In conclusion, this study shows that replacing
low quality grass silage with 33 or 67% of maize
silage linearly increases the diet FM intake and di-
gestibility of DM, OM, ADF, starch, D-value and

N intake in wether sheep. These linear effects on
intake and digestibility suggest that there were no
interactions between the forages, but the presence
of quadratic effects in the diet FM, DM, and OM
intake, digestibility of DM, OM, NDF, ADF, CP,
starch and D-value as well as in N intake and N
balance proved the existence of associative effects.
A positive associative response of the two forages
was recorded for all the measured parameters as
expected, probably due to the fact that MS com-
plements GS well and that the replaced GS was of
lower quality than the included MS.

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Feeding value of low quality grass silagesupplemented with maize silage for sheep
Introduction
Material and methods
Results
References