SOIL SCIENCE NEWS AND VIEWS
Vol. 17, No.1, 1996

     Previous studies conducted in Kentucky have shown that hairy
vetch is an agronomically viable winter legume cover crop for no-
till corn production.  However, to fully realize the benefits of
this cover crop it is necessary to allow enough time for it to
accumulate biomass and nitrogen (N) before no-till corn is
planted.  Thus, early corn planting may not always be feasible.
Since delaying corn planting after May 15 in Kentucky usually
leads to a decline in corn grain yields, the main question
addressed in this study (one for which little information was
available) was whether the benefits provided by hairy vetch could
offset the consequences of delayed corn planting.
     This study was designed to answer the following questions:
(1) Do winter cover crops influence the impact of delayed corn
planting on grain yield? (2) What effect(s) do winter cover crops
have on N mineralization early in the corn growing season before
fertilizer N is applied? (3) Does hairy vetch provide benefits
other than a supply of N? (4) What economic benefit would a corn
grower expect from using hairy vetch in a no-till corn production
system? (5) What effect(s) do winter cover crops have on the
response of corn to fertilizer N application?

RESULTS

     This study was conducted on a Maury silt loam at UK's
Spindletop Experiment Farm at Lexington for four years (1992-95).
Corn was grown continuously and was planted at three different
dates (about April 28, May 15, and May 31) each year.  Corn was
planted following no winter cover (winter weeds only), hairy
vetch, or winter wheat at each planting date, and the crop in
each cover system was fertilized with four rates of N (0, 60,
120, and 180 lb N/acre as ammonium nitrate) about four weeks
after planting.  The function describing the corn grain yield
response to the fertilizer N was calculated for each cover
system, at each planting date, and for each year.  Functions for
the winter weeds (no cover) and winter wheat cover systems were
not different , so a single function relating grain yield to
fertilizer N was calculated to describe those two cover systems.
From such functions the optimal fertilizer N rate (the point at
which the cost of the last pound of N was just paid for with corn
grain) was determined.  the corn grain yield at that point was
also found.  Each of these parameters was calculated for each
year and then averaged over the four years of the study (Table
1).
     As expected, corn following winter weeds or wheat required
greater amounts of fertilizer N to achieve optimal yields than
when following vetch (Table 1).  Corn following either winter
weeds or wheat gave a greater yield response to added N (bigger
yield increment with each unit of fertilizer N) than corn
following hairy vetch.  This indicated a lower supply of soil N
under winter weeds or wheat as compared to vetch.  Corn grain
yields in both winter weeds and wheat cover systems tended to be
highest when planting was done in mid-May (Table 1).  Though the
yield decline was modest, there was a trend for reduced optimal
yield at the latest planting date when corn followed winter weeds
or wheat.  Unexpectedly, this was not true when corn followed
vetch, where yield actually rose with delayed planting.  The
results showed that corn planted following hairy vetch could be
delayed by up to four weeks (until June 1) from the start of the
corn growing season (May 1), without causing yield loss.
However, growers should remember that in a cold, dry spring, when
growth of hairy vetch is limited, some of this benefit may not be
realized.  Both the winter weeds (no cover sown) and wheat
systems behaved similarly in many respects.  Therefore, it does
not appear to be worth investing in seeding winter wheat just for
cover in this continuous corn production system.
     Net soil N mineralization rates in the first five weeks
after cover crop kill (corn planting) were higher under hairy vetch than
 under either winter weeds or wheat data not shown).  Thus, hairy 
vetch improved soil N supply to corn early in the season, prior to fertilizer N
application.  The nitrogen fertilizer equivalence (NFE) to using
hairy vetch can be calculated by subtracting the optimal N rate
for corn following vetch from that for corn following winter
weeds or wheat.  The NFE values ranged from 26 lb
N/acre at late April planting dates to 41 lb N/acre at late May
planting dates (average of 32 lb N/acre).  Though greater NFE
values with greater vetch growth and delayed planting were
expected, the amount of that increase was smaller than expected,
probably because of an unexpected decline in the optimum N rate
with delayed planting in both the winter weeds and wheat cover
systems. It is not clear why the optimal fertilizer N rate in
these two systems declined from 164 to 142 lb N/acre when
planting was delayed until at least mid-May.
     Grain yield at the optimal fertilizer N rate following vetch
was always superior to that following winter weeds or wheat.
This difference (a form of "rotation effect") ranged from 3
bu/acre with mid-May planting to 14 bu/acre with late May
planting (average of 8 bu/acre).  Clearly, greater soil N supply
after vetch did not fully account for the greater corn yield
observed.  This indicates that hairy vetch provided benefits
above and beyond a supply of N, perhaps additional soil moisture
conservation under the vetch mulch and/or other positive
"rotation effects", to the corn which followed.  Soil moisture
was often higher under corn following hairy vetch (data not
shown).

SUMMARY

     The total value to planting corn after hairy vetch can be
calculated (NFE x price of N per pound) + ("rotation effect"
yield x price of corn per bushel).  Average corn and fertilizer N
prices for the 1992-94 production and marketing years were
$2.39/bu and $0.21/lb, respectively.  Total value averaged nearly
$26/acre (range of about $13/acre with mid-May planting to
$42/acre with late May planting).  Commodity and input prices,
as well as the NFE and "rotation effect" yields, can vary, of
course.
     The observed benefits of hairy vetch (N supply, soil
moisture improvement, and rotation effect) are probably the most
important criteria a farmer can use in deciding whether to
include this winter legume in a no-till corn production system.
It might be misleading to judge the usefulness of hairy vetch
based on its N supply characteristic only.  When all the benefits
provided by hairy vetch are considered, it is evident that this
cover crop fits well in a no-till corn production system in
Kentucky.  The hairy vetch cover system works particularly well
when the spring season is warm and moist enough to encourage
rapid growth of the cover crop.
     The current recommendation for Kentucky is that corn
planting should be completed before May 10 or May 20 in the
western and eastern parts of the state, respectively.  However,
this study shows that there is a period of about four weeks (from
1 May until 1 June) within which corn can be no-till planted into
a hairy vetch cover crop without incurring significant yield
loss.


Table 1.  The optimum fertilizer N rate, and the corn grain yield
at that N rate, as a function of winter cover system and corn
planting date (ave. 1992-95).
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     Winter             Planting      Optimum            Grain
     Cover                Date         N Rate            Yield
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                                     lb N/acre          bu/acre

weeds/wheat           late April      164                132
                      mid-May         142                145
                      late May        142                140

hairy vetch           late April      138                139
                      mid-May         113                148
                      late May        101                154
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