192
Intoxications of Cattle by Sudan Grass
TOXICITY IN RELATION TO STAGE OF GROWTH
It has been stated often that the crop in flower
or seed may be fed off with impunity, but clinical
observation proves this assertion to be utterly
erroneous and dangerously misleading. The clini-
cal observer does not quickly forget the occasions
when he arrives at a farm to find a sudan grass pad-
dock of several acres with most of the crop in flower
and seed, and a dozen or more dairy cows dead and
dying after a quarter of an hour's grazing. One
good example of such a crop was No. 29 in the
table. It has been argued that it is the immature
plants that are toxic, but against this one has been
careful to submit leaves from the mature forms for
analysis. Moreover, the farmer who risks his
cattle on unsound advice is not concerned with any
neat distinctions, and if the crop is well grown and
in seed then that is good enough for him, but to his
undoing.
It will not be denied that immature crops are
probably more toxic than mature crops, but it
must be stressed that olinical observation indicates
that the plant is never safe.
It seems fairly obvious that some disturbance to
the even growth rate of the plant is the main factor
in rendering it toxic. It has often been stated
that a stunted crop is the more dangerous, but
such has not been borne out in these observations.
How intimately the toxicity factor is bound up
with seasonal conditions will be shown in the table
and in the next paragraph.
THE EFFECT OF SEASONAL CONDITIONS
Sudan grass mortalities always occur in waves
and one can always anticipate when they are likely
to occur. Moreover, when one accident occurs it
is usually followed by a number of others in the
same or widely separated districts; and at the
same time sheep mortalities may occur as the result
of the ingestion of well-known cyanogenetic plants
such as Cynodon sp., Chenopodium carinatum, and
Euphorbia drummondii. This widespread occurr-
ence and diversity of mortalities in sheep and cattle
as a result of eating cyanogenetic plants is not just
chance, but is due to the occurrence of seasonal
conditions which favour the production of cyano-
genetic glucoside in the plant.
Sudan grass is usually planted in the spring
(October) and with sufficient soil moisture or rain
a good crop results. This being a winter rainfall
district, it is normal for the summers to be intensely
hot and dry, and under these conditions the crop
does not grow, and even though it be sparse and
stunted in the paddock and cattle have access to
it, no ill-effects follow. If, however, the crop
survives till mid-summer and rain occurs under
conditions of high temperatures, the sudan grass
grows prolifically and the common practice among
dairy farmers is to put their cows on to the crop
for an hour or so each day. Under such a system
the crop holds its growth, stools out freely and milk
production increases. These are the seasonal
conditions which induce toxicity of crops and
mortalities in cattle; but yet another factor seems
to be necessary and this will be discussed under the
next heading.
A glance at the table will show the years when
trouble has occurred. They were February
March, 1931, December, 1931, January, 1932, then
nothing occurred until the series commencing in
December, 1934, and ending in January, 1935.
There was thereafter again a period of immunity
from trouble till December, 1937, when followed
another series of losses which ended the next
month.
THE PART PLAYED BY GRASSHOPPER INVASIO
The seasons already referred to as favouring the
good growth of sudan grass are also those which
favour the hatching, swarming and invasion of the
district with grasshoppers.
These pests advance up the valleys of creeks and
rivers and attack anything that is green. Sudan
grass they devour in any stage of its growth and
remain on it long enough only to eat the crop to the
ground, so that viewed from a little distance the
paddock appears bare. To do this the swarms
may remain from two to ten days and then pass on.
During the visitation many of the plants are eaten
into the ground and are completely destroyed.
In a showery summer, some rain falls after the
crop has been eaten back, warmth continues and
the re-growth is very rapid. Meanwhile, owing
to a lack of feed and in order to allow the sudan
grass an opportunity to recover, the cattle are kept
out of the paddock for a period, generally in the
region of two weeks. The first grazing of the crop
after this spell is the fatal one and usually symp-
toms are manifest in about ten minutes. In that
time sufficient of the crop may be eaten to kill half
the herd. A reference to the table will show how
almost invariably the grasshoppers have been
associated with these mortalities, so that one has
come to regard these pests as almost an essential
factor. It is considered, however, that their ac-
tion must be a mechanical one only, and that if
some other factor was responsible for cutting the
crop in precisely similar circumstances and the
same favourable conditions for re-growth occurred
the crop would doubtless be just as toxic as if eaten
back by grasshoppers. Factors which might occur
in practice are mowing and heavy stocking. The
broom millet crop referred to in No. 35 of the table
is the only one of the series known to have been cut.
Cutting right back by grazing seldom occurs, it
being the usual practice to allow a limited daily
grazing ; but when such has occurred, the herd is