Replacing summer fallow with an annual legume is an option being
considered by some producers. Possible benefits include reduction of fertilizer inputs
(due to biological fixation of nitrogen), reduction of pesticide inputs (due to disruption
of pest life cycles), improved labor and machinery efficiency, and risk reduction through
crop diversification.
This study was started in 1995 at the Williston Research Extension
Center. The crop sequences being compared are wheat grown continuously or alternating with
summer fallow, safflower, pea, grain lentil, or forage lentil. For each legume species,
both harvested and green-manured treatments are included.
Two sets of the continuous wheat treatment as well as each of the green
manure treatments were included to allow for different N fertilizer rates on the
subsequent wheat. The second continuous wheat treatment received only 30 lb/a of N,
rather than the recommended 90 lb/a. (This lesser rate was the determined by using N
recommendation for wheat following fallow). The second set of green manure treatments
received 90 lb/a of N, rather than the recommended 30 lb/a. (This greater rate
was the determined by using the N recommendation for continuous wheat). All the other
treatments received the recommended N rates based on soil testing.
Above ground biomass was determined for all crops at the time of green
manuring and at harvest. Protein content was determined for wheat, and oil content was
determined for safflower.
Dry soil conditions this spring reduced the germination of
shallow-seeded crops (lentils and safflower) whereas the germination of deeper-seeded
crops (pea and wheat) was unaffected. Forage lentil biomass at green manuring and harvest
was less than the 3-yr average (Table 1). Grain lentil biomass was greater than the
3-yr average at green manuring but less than the 3-yr average at harvest. Pea biomass
production at both green manuring and harvest was greater than the 3-yr average. In prior
years, legume biomass increased greatly between green manuring and harvest. This year,
only pea biomass increased – forage lentil remained constant (within sampling error),
and grain lentil biomass actually decreased during this time.
Pea grain yield was the greatest ever in the four years of this study
(Table 2). Unfortunately, grain yields of lentil and safflower were at an all-time
low. This difference among crops may be due the differences in germination and the
consequential negative effect of greater weed competition.
Wheat yields in 1998 were 5.5 bu/a greater than the 3-yr
average (Table 3). Greatest wheat yields were obtained following the fallow and
green manure treatments. The lowest wheat yields occurred in the continuous wheat
treatment with insufficient N (30 lb/a) and with wheat following a harvested legume
or safflower.
For the continuous wheat treatments, applying
insufficient N significantly reduced yields both this year and last year. For wheat
following the green manure treatments, applying excess N did not significantly increase
yields. This has been true for all three green manure treatments every year of this study.
The finding that insufficient N generally reduced yields and that excess N never increased
yields supports the validity of fertilizer recommendations based on soil testing.
Protein content this year was less than the 3-yr
average. Protein content was the least in the continuous wheat with the 30-lb/a nitrogen
application, and greatest in the wheat following a legume (either harvested or green
manured) with the 90-lb/a N application. Applying 90 lb/a N to wheat following green
manure versus the recommended 30 lb/a increased the protein content in two of the three
cases. Excess N increased the protein content of wheat following green manure in 1996 but
not 1997. This suggests that although protein content is maximized at a greater N rate
than is yield, applying more N than recommended inconsistently increases protein content,
and may not be economically beneficial.
This study is being continued to determine if the
observed trends continue and if long-term benefits such as improvement in soil quality are
obtained.
