Íslenskar landbúnaðarrannsóknir - 01.09.1982, Blaðsíða 63

SPRING GRAZING 61
EFFECT OF UNTIMELY SPRING GRAZING ON CARRYING CAPACITY
AND ANIMAL GAINS
Several studies in Iceland have demons-
trated that heavy or untimely spring
grazing of rangelands and hayfields can
reduce plant productivity (Jonsson 1974;
Arnalds 1981; Geirsson 1981; Óskars-
son 1982). Although the qualitative
relationship between seasonal plant pro-
ductivity and rangeland carrying capacity
is generally well known, there is little
quantitative data available to clarifyjust
how much herbage should be allowed to
accumulate in spring before grazing can be
safely initiated. Good management does
not imply that we should manage plants in
such a way that carbohydrate reserve
levels and leaf areas are maximized. But,
on the other hand care should be taken
not to exceed critically low levels. By
deferring grazing in spring until plants
have sufficient leaf area to be in a positive
carbon balance, plant vigor and pro-
ductivity can be maitained at higher
levels throughout the remainder of the
growing season. As a result, the overall
carrying capacity of the range will be
enhanced.
In addition, animal performance is
often quite poor when grazing is initiated
too early in the growing season. Table 1
summarizes the results obtained in an
Ireland study (Collins et al. 1977) in
which animal performance was evaluated
for various spring turnout dates. These
data illustrate that even a short delay in
initiation of grazing can have dramatic
positive effects on daily animal gains. On
the other hand, forage quality is highest
earliest in the growing season. Delaying
the initiation of grazing for too long
might then result in reduced animal
gains. Thus, it is desirable to accurately
ascertain plant growth in the spring, and
to discover the point at which animal
performance can be opttimized with
minimal detrimental effects to key forage
plants. While the results from the Ireland
study (Table 1) may hold conceptually
for Icelandic ranges, they should not be
interpreted literally, for Icelandic ranges
are generally much lower in productivity.
Further, the time at which plants can be
best utilized in the spring will differ ffom
year to year as climatic conditions vary
from locality to locality. Thus, the vigor
of key forage species should be monitored
and turnout dates and stocking rates
adjusted accordingly.
TABLE I.
Forage standing crop (dry weight) and weight gain of cattle on Irish pastures in early spring
1. Tafla
Uppskera á rót (þurrvigt) og þyngdaraukning nautgriþa á írsku graslendi í byrjun gróanda
Starting date of grazing
Upþhaf beitar
Apr 1 Apr 8 Apr 15 Apr 22
Forage standing crop (kg/ha) Uppskera á rót (kg/ha) 254 508 1261 1608
Animal live weight gain (kg/ha) Þyngdaraukning dýra (kg/ha) 833 976 1046 891
% animal weight gain relative to April 1 Hlutfallsleg þyngdaraukning miðað við 1. apríl 100 117 126 107
(from Collins et al. 1977)