PCC July-August 2020

Weed Control in Lettuce
A Comparison of Various Weed Management Strategies and Costs
By RICHARD SMITH | UCCE Vegetable Crop and Weed Science
Farm Advisor
Economical and successful weed
control in lettuce can be accomplished
by utilizing key cultural practices,
cultivation technologies and herbicides.
Planting configurations vary from
40-inch wide beds with two seedlines to
80-inch wide beds with 5 to 6 seedlines.
Recent studies of weeding costs for lettuce
ranged from $454 to $623/A for 80-
inch wide beds with 5 seedlines of head
and 6 seedlines of romaine hearts lettuces,
respectively (see coststudies.ucdavis.edu/
en/current/commodity/lettuce/).
Weeding costs included the following:
Herbicide applied in 4-inch wide bands
over the seedlines, cultivation, auto thinning
using a fertilizer to kill unwanted
lettuce plants and hand weeding/double
removal. The costs for auto thinning also
include fertilizer costs, which can satisfy
the need for the first fertilizer application.
Significant weed control is accomplished
by practices that occur before the crop
is planted. For instance, weed pressure
is affected by prior crop rotations and
how much weed seed was produced in
them. The weeding costs given above are
rough averages. If weed pressure is light,
weeding costs can be lower, but if weed
pressure is high, weeding costs can be
much higher. In the Salinas Valley, good
management of weeds is possible with
rotational crops such as baby vegetables
(spinach, baby lettuce and spring mix)
because they mature in 25 to 35 days and
don’t allow weeds to set seed. Long-season
crops such as pepper and annual
artichokes allow multiple waves of weeds
to germinate which are difficult to see
and remove once the plants get bigger.
Preirrigation is standard practice to
prepare the beds for planting. It stimulates
germination of a percentage of
weed seeds in the seedbank, and they are
subsequently killed by tillage operations.
Studies have shown that preirrigation
followed by tillage lowers weed pressure
to the subsequent crop by about 50%. In
organic production, pregermination is
one of the most powerful practices for reducing
weed pressure, and if time allows,
it can be repeated to further reduce weed
pressure.
Preemergence Herbicides
There are three pre-emergence herbicides
available for use in lettuce production:
Balan, Prefar and Kerb. Balan and Prefar
provide good control of key warm season
weeds such as lambsquarters, pigweed
and purslane, as well as grasses (Table 1,
see page 5). Kerb is better at controlling
mustard and nightshade family weeds
such as shepherd’s purse and nightshades.
Balan is mechanically incorporated
into the soil and Prefar and Kerb
are commonly applied at or post planting
and incorporated into the soil with germination
water.
Kerb is more mobile in water than
Prefar which can lead to issues with its
efficacy. Often 1.5 to 2.0 inches of water
are applied with the first irrigation to
germinate the crop which can cause Kerb
to move below the zone of germinating
weed seeds, especially on sandy soils. For
instance, Kerb is capable of controlling
purslane however, its efficacy can be low
on sandy soils due to its movement below
the zone of germinating weed seeds with
the first germination water. Prefar does
not leach as readily as Kerb and that
is why these two herbicides are often
mixed in the summer to control purslane
(Figure 1).
In the desert, the use of delayed applications
of Kerb has been used for many
years. Due to the large amounts of water
that are applied to keep the seeds moist
Drip germination in lettuce has resulted in
fewer weeds than sprinkler irrigation (photo by
Marni Katz.)
and cool, Kerb is applied in the 2nd or
3rd germination water, approximately 3
to 5 days following the first water, just
prior to the emergence of the lettuce
seedlings. The amount of water applied
in the second and third irrigation is less
than the first application and therefore
does not push the Kerb as deep in the
soil. Although the Salinas Valley is
cooler than the desert, evaluations here
have also found delayed applications to
improve the efficacy of Kerb (Figure 2,
see page 6). These data illustrate the loss
of control of purslane by Kerb when applied
before the first germination water,
as well as the improvement in efficacy
that results when applied after the first
germination water. It also illustrates the
role that Prefar plays in the control of
purslane when the efficacy of Kerb is reduced
by being pushed too deep. Clearly,
there is benefit from applying the Kerb
in the 2nd or 3rd germination water because
it helps to keep it in the zone where
weed seeds are germinating.
Figure 1. On left: Kerb at 3.5 pints/A applied at
planting; On right Kerb at 3.5 pints/A
+ Prefar at 1.0 gallon/A applied at planting. The
main weed is common purslane which
was not controlled by Kerb because it was
pushed below the zone of germinating
weed seeds by the germination water (photo
courtesy R. Smith.)
4 Progressive Crop Consultant July / August 2020