Mechanical management of weeds - Ecological Management of Agricultural Weeds - page 180

Table 4.4. Weed control and soybean yield with timely and untimely rotary hoeing and
various soil conditions

Weed dry weight Soybean yield

Hoe

Treatment passes kg ha 1 % control kg ha 1 % ofmax

Timely hand weed 2 0 100 2620 100

Timely 3 530 72 2280 87

Untimely 2 1160 38 1950 74

Timely,wet after 3 1250 33 2150 81

Untimely,wet after 2 1390 25 1880 71

Timely,wet before 3 1250 33 2420 92

Untimely,wet before 2 1660 11 1880 72

Weedy check — 1860 0 1610 61

Source: Adapted from Lovely,Weber & Staniforth (1958).

two untimely hoeings reduced it only 38% (Table 4.4). Similarly, Mulder &
Doll (1993) found that three rotary hoeings controlled weeds better than two
hoeings, and Mohler, Frisch & Mt. Pleasant (1997) found one to two hoeings
inadequate in some years. However, VanGessel et al . (1995 a ) found one well-
timed rotary hoeing as good as two.Mohler & Frisch (1997) found a rotary hoe
about as effective as a spring tine weeding harrow for weed management in
oat.
Wet soil conditions reduce the effectiveness of rotary hoeing (Table 4.4).
Wet ground limits soil movement by the implement,and rainfall or irrigation
soon after rotary hoeing probably reduces the percentage of weeds that die by
desiccation.
Harrows vary greatly in design, but all consist of a frame with many down-
ward pointing,small diameter tines.Although chain harrows and spike-tooth
harrows are still successfully used for weed control, spring-tine harrows are
rapidly replacing these more traditional designs. The tines on spring-tine
harrows are either spring steel wires (typically 4-7 mm diameter),or else rigid
metal fingers attached to the frame with a spring.Their popularity as weeding
tools comes from a greater ability to adjust down pressure and hence aggres-
siveness, coupled with the ability of the tines to spring over or around well-
rooted crop plants. Nevertheless, comparison trials have not demonstrated
superiority of the spring-tine design over older types of harrows (Rasmussen,
1992 a ; Wilson,Wright & Butler, 1993).
In cereals, harrows are most commonly used pre-emergence or at emer-
gence, and again when the crop has two to three leaves. Cultivation between
these stages will usually bury too many crop plants. Harrows are also some-