Summer 1994 (v6n3)
Sills Farms long-term rice experiment: Summary.
G. Stuart Pettygrove
Article written for Sustainable Agriculture/Technical Reviews.
1994
In a 15-acre field experiment started in 1988 at Sills Farms in
Sutter County, we have shown that winter cover cropping in a continuous,
high-yielding rice rotation provides a significant benefit (table
1). Improvements in rice yields with the cover crop were found
with less-than-ideal conditions (acid soil pH 4.5, poor drainage,
presence of unchopped rice straw) and occurred across the three
different residue management systems: fall disced, fall burned,
and spring disced.
Conventional practices have been used on this field and rice has
been planted each year. The system is, therefore, quite different
from the typical organic rice system in which one crop of rice
is followed by one or more years out of rice with cover cropping.
The production of the cover crop has not required any special
equipment or inputs other than the seed for the cover crop.
During three years (1990-1992), cover cropping with purple vetch
reduced the nitrogen requirement of the rice by 60 to 105 pounds
per acre. These figures were determined by fertilizer nitrogen
rate plots established each year in the experiment's main plots.
This reduction in the amount of fertilizer needed represents a
savings of between 2 and 3.5 million BTUs of energy per acre annually
(equivalent to 1425 gallons diesel fuel per acre). The resultant
energy savings outweighs the small amount of energy required to
produce and harvest the cover crop seed and broadcast it in a
rice field.
The profitability of green manuring is also being assessed. Calculating
the difference in net income between the green manure and non-green
manure rotation depends on a number of factors including the cost
of vetch seed and the seeding rate. If the grower produces his
or her own vetch seed, the land charge or opportunity cost must
also be factored in. In our calculations, we have assumed that:
1) purple vetch seed was produced by the grower at a cost of 10
cents per pound including land charges; 2) vetch was planted as
a green manure crop at 50 pounds per acre; 3) rice was fertilized
each year in all plots at the optimal nitrogen rates determined
in the fertilizer sub-plots. With these assumptions, the cost
of green manuring comes to $18 per acre.
The fertilizer value of the nitrogen contained in the vetch ranged
from $4 to $22 per acre (1990-1992). In 1993, wet winter conditions
resulted in extremely poor vetch growth, so little nitrogen value
was realized. In reality, a farmer would respond to this poor
growth by fertilizing rice at an appropriate rate.
In all three straw management systems, we have observed a slightly
better maximum yield on the vetch plots than on non-vetch plots
(table 1). This effect is not related to nitrogen levels since,
even at the highest nitrogen rates, grain yield on non-cover cropped
plots did not reach maximum yields on the cover cropped plots.
We have not so far observed any obvious effects of green manuring
on rice diseases or on rice straw decomposition, and cannot otherwise
explain the higher yields on the vetch plots. When these higher
yields are combined with the nitrogen fertilizer effects, the
overall net gain (of green manuring compared to winter fallow)
in four years is $6 per acre on the fall disced system, $47 for
the spring disced system and $104 for the burned system.
This research was funded through grants from the Rice Research
Board, the California Energy Commission, and UC SAREP. Cooperating
investigators include Stuart Pettygrove and Kate Scow (UCD Land,
Air and Water Resources), Jack Williams (UC Cooperative Extension,
Sutter-Yuba) and Jim Hill (UCD Agronomy and Range Science).
For more information write to: G.S. Pettygrove, Department of
Land, Air and Water Resources, University of California, Davis,
CA 95616.
(DEC.519) Contributed by G.S. Pettygrove
| Table 1. Maximum rice yields and required fertilizer nitrogen rate. Sills Farms, 1990-1993. | |||||
| 1990 | 1991 | 1992 | 1993 | 4-year average | |
| No vetch | |||||
| Fall burn | 90.7(104) | 104.3(168) | 101.8(120) | 84.8(60) | 95.4 |
| Fall disc | 93.7(118) | 107.6(204) | 104.1(90) | 92.1(90) | 99.4 |
| Spring disc | 92.8(124) | 106.8(>180) | 102.5(90) | 86.8(90) | 97.2 |
| Vetch | |||||
| Fall burn | 95.3(30) | 100.7(60) | 105.2(30) | 92.6(60) | 98.5 |
| Fall disc | 95.2(30) | 112.1(114) | 105.2(30) | 88.0(90) | 100.1 |
| Spring disc | 95.3(30) | 106.9(158) | 104.9(30) | 89.6(60) | 99.2 |
| Maximum yields were determined by linear regression of yields vs. fertilizer rate. | |||||
| N rates in pounds per acre required for maximum yields are shown in parentheses. | |||||
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