Íslenskar landbúnaðarrannsóknir - 01.09.1982, Page 6

4 ÍSLENZKAR LANDBÚNAÐARRANNSÓKNIR
ber of growers on the sandy soils of
southern Iceland who are occasionally
resting their helds from potato cultivation
for 1 to 3 years during which period the
land is either left fallow or used to grow
other crops.
Potato growing is economically impossi-
ble without the use of a herbicide. Chick-
weed (Stellaria media) is a widespread
problem. About 16 years ago a herbicide
whose active ingredient is linuron was
introduced here in Iceland, but in the
preceding 3—5 years a monolinuron herbi-
cide had beed used. The amount oflinuron
used initially was rather high, 1—1.5 kg/ha.
But the growers gradually established that
the amount could be reduced on sandy soils
without reduction in eífectiveness so the
amount now used there has fallen below 0.5
kg/ha. At present the recommended dos-
ages in Iceland are: loam 1.25 kg/ha, sandy
loam 1.0 kg/ha and sand 0.5 kg/ha.
Since the introduction of linuron, no
other herbicide has been used in potato
cultivation in Iceland. It is only about ten
years since a register of permissible pesti-
cides was initiated and in that period of
time linuron has been the only registered
herbicide suitable for potato growing.
There has been no requirement to find an
alternative because the effect oflinuron has
been satisfactory in most cases. There has,
however, in recent years been a growing
interest in another herbicide, because
some growers have problems with other
weeds which are not controlled suíficiently
by linuron (e.g. Polygonum aviculare).
Linuron (N- (3,4 dichlorophenyl) -
N’methoxy-N’-methyl urea) is one of the
group of herbicides known as substituted
urea compounds. It is known to be readily
translocated in the xylem ofplants, closely
following the transpiration stream to the
leaves where it affects photosynthesis.
Visual symptoms of linuron damage
usually involve chlorosis of the leaves (see
plate 1), stunted growth and occasionally
necrosis. Most annual weeds, which can be
a problem in intensive vegetable cultiva-
tion, are relatively sensitive to linuron but
certain crops have been shown to be
tolerant to this herbicide. Carrot and
parsnip are particularly tolerant species
but potato has slightly less linuron resist-
ance. On the other hand, lettuce, turnip
and wheat are linuron sensitive (C averley
1978). By use of radio assay techniques it
has been shown that when carrot or
parsnip is grown in a linuron treated
medium, more than 70% of the absorbed
herbicide remains in the root system
whereas in the susceptible turnip and
lettuce, over 60% is translocated to the
leaves (Walker 1971). These results sug-
gest that the reason for the resistance to
linuron in some crops, is their comparative
inability to translocate the herbicide to the
aerial shoots. Potato was not, however,
included in this investigation.
The exceptionally intensive use of
linuron in Iceland has been a matter of
concern for some time. Reports from other
Scandinavian countries (Steckó and
Andersson 1973, Andersen and
Samuelsen 1974) indicate that there is a
danger of persistance of urea herbicides
particularly where the soil temperature is
low. The purpose of this survey was to
investigate the possibility that linuron is
accumulating in our fields and leaving
residues in the potatoes which could
possibly have a detrimental effect on yield
and eventually cause us to consume
unnecessary amounts of this herbicide.