Crop diversification for weed management - Ecological Management of Agricultural Weeds - page 334

germination of many weed species due to changes in spectral composition (see
Chapter 2).Reduced soil disturbance during the time when a perennial forage
crop occupies a field also suppresses weed germination relative to tilled condi-
tions (Roberts & Feast,1973).For all of these reasons,few new weed seeds may
be added to the soil seed bank during the forage phase of a crop rotation.
Weed seed survival in soil can be higher in untilled forages than tilled crops
(Warnes & Andersen, 1984; Lueschen et al ., 1993), but the combined effects of
reducing seed inputs and maintaining a moderate level of seed mortality can
result in substantial reductions in weed density. Eighty-three percent of the
Manitoba and Saskatchewan farmers surveyed by Entz, Bullied & Katepa-
Mupondwa (1995) reported fewer weeds in cereals following perennial
forages (especially alfalfa) than following cereal crops; 67% reported higher
grain yields following forages than following cereal crops. The farmers noted
that annual grass and annual broadleaf weed species were particularly well
controlled through the use of perennial forages.
In field experiments, weed seed populations were found to decline by 99%
for Avena fatua following three years of perennial ryegrass/white clover sod
(Wilson & Phipps, 1985), 47% for Abutilon theophrasti following two years of
alfalfa (Lueschen et al ., 1993), and 47% for Brassica kaber following one and a
half years of smooth bromegrass (Warnes & Andersen, 1984). In each case,
maintenance of the forage stands for longer periods of time had little addi-
tional effect on weed seed mortality. Inclusion of perennial forage crops in
rotations should therefore be expected to reduce, but not eliminate, seed pop-
ulations of many weeds typically found in arable crops.
If forage management practices are improperly timed relative to weed
growth and reproduction, weed seed density may increase more during the
production of forages than annual crops. In one of two years, Clay & Aguilar
(1998) observed higher weed seed densities in soil following alfalfa than con-
tinuous maize, and attributed this difference to seed shed that occurred
between the penultimate and final alfalfa harvests. However, weed seedling
densities in maize were consistently lower following alfalfa than following
maize (Clay & Aguilar, 1998), suggesting that seedling establishment may
have been more important than seed survival for regulating weed density.
Differences between the two cropping systems in soil chemical, physical, or
biological characteristics may have been responsible for differences in weed
emergence (see Chapter 5).
While perennial forage phases of a rotation may reduce infestations of
certain weed species, they may also allow for increases in populations of
others. Ominski, Entz & Kenkel (1999) measured weed densities in Manitoba
wheat, oat, and barley fields that were occupied the previous year by either
alfalfa hay or cereals.Densities of Avena fatua , Circium arvense , Brassica kaber ,and