Research Results - (PDF, 101 mb) - USAID

Grazing and Animal Nutrition Studies
in the Alfa Grass Ecosystem
Wax and Silica Content of Alfa Grass in Relation
to Defoliation
The coarse alfa grass (Stipatenacissima)
that dominates the physiognomy and productivity
of a major part of semi-arid steppe vegetation
in North Africa is well known to be of
relatively low nutritional quality and palatability,
as described in earlier SR-CRSP reports. An
experiment conducted at the El Faijah and
Plaine de l'Aarid research stations in 1990
examined the theory that the low palatability of
the grass was due to defenses against defoliation
in the form of wax and silica in the leaves. Wax
and silica are known to depress digestibility and
therefore relative palatability of forage grasses.
It was hypothesized that silica and wax content
of the alfa grass leaves increased in response to
defoliation. On March 31, four clipping treatments
were applied, plus a control. Plants were
clipped to the base of the shoot, to the ligule at
the junction of leaf sheath and blade, or down to
10 cm above the ligule. Regrowth, or new
growth in the case of the control, was sampled
at 46, 92, 135, and 179 days after clipping and
measured for crude protein (CP), ash, the Van
Soest fractions of acid detergent and neutral
detergent fiber (ADF and NDF) and acid detergent
lignin (ADL), as well as for silica and wax
content. The experiment was conducted on 3x3
m plots with 12 replications at each site. The
data are summarized in terms of percent dry
matter (%DM), and significant differences are
reported at the 5%level. The results of this
study are being prepared for publication with
Dr. Hamid Narjisse as senior author.
Samples at El Faijah tended to be higher
in wax content and lower in silica content: 1.50
vs 1.37, and 0.99 vs 1.43, respectively, for El
Faijah and Plaine de l'Aarid when all samples
are combined. For other nutritional parameters
there were generally only slight differences
between sites. Ash content across all samples
and treatments was lower at El Faijah (3.45 vs
3.87 %DM) while NDF and ADL were higher
(78.03 vs 77.23, and 7.44 vs 6.73, respectively),
The effect of sampling time was expressed most
strongly in crude protein and to a lesser degree
in NDF, as shown in table I. ADF was lowest at
the 46 day sample (43.80%DM) compared to
later sampling dates (48.48 for the average of the
92, 135, and 179 day samples), and similarly
ADL was lower at the first sampling time
(6.41 %DM) compared to the other times (7.31 for
the average of the three), when analyzed across
all treatments and both sites.
Table 1. Trends in crude protein (CP) and
neutral detergent fiber (NDF) over the four
sampling times, all treatments and both sites
combined. Data are expressed as percent drt
matter.
days after clipping treatment
46 92 135 179
CP 10.96a 7.64b 6.75c 5.95d
NDF 75.76a 80.08c 78.66b 77.64b
Means followed by the same letter in a row are
not significantly different (p