Vol. 16—1962 - NorthEastern Weed Science Society

50
In the third field test, in which no crops were included, a somewhat
di~terent' approach was taken. The test area was thoroughly cultivated with
a rotovator,and then divided into four equal blocks or replicates. Bands
4 16" wide were treated with dimethyl tetrachloroterephthalate at 6 and 2
:l,b./acre, sprayed. in a north-south direction, the length of the replicate,
each replicate being treated separately. Also, untreated north-Bouth barns
of the same width were included -in each block. Baoos of the other herbicides,
5 16" wide were then applied in an east-west direction, the width of the
replicate, crossing the bands of dimethyl tetrachloroterephthalate at right
angles. Dosages applied werel'Crro at 4 and 2 lb./acre, NPAat 4 and 2
lb./acre and.2,4-D'at 2 and llb./acre. Again, each block was treated
separately and untreated east-west bandswereinc],uded. This procedure
resulted in a series of 4 16" X 5 16" subplot~ containing all combinations of
dimethyl tetrachloroterephthalate with all dosages' of the other herbicides.
In addition, subplots containing each dosage of each herbicide alone, as well
as untreated oheoks were formed.
Plot size in the crabgrass oontrol test was 7' X 7' with three replicates
in a randomized block design. Combinations were applied as tank mixes
in 100 gallons of water per acre. Dimethyl tetrachloroterephthalate at 10
and 5 lb./acre, 2,4-D at 0.5 lb./acre, NPAat 10 lb./acre, ohlordane at 40
lb./acre, dimethyl tetrachloroterephthalate + 2,k-P at 10 + 0.5 lb./acre,
dimethyl tetraohloroterephthalate + NPA at 10 + 10-and 5 + 10 lb./aore, and
dimethyl tetrachloroterephthalate + chlordane at 5+ 40 lb./acre were
included in the test.
The amount of weed oontrol whioh could have been expeoted from the combinations
was oalculated and is reported in eaoh table. The following
formula, whioh was derived from the literature on inseotioide synergism (1),
was used to oalculate expeoted weed oontrol assumiDg oomplete independent
action of the components in the combinations I
x = %weed oontrol by h6rbioide A at p pounds per aore
y = %weed oontrol by herbioide B at r poullds per aore
E = expeoted %weed control by A + B at p + r pounds per acre
E=x+y-~
This equation is oonsidered to be valid only when applied to a given
speoies. When oombinations are evaluated against a population of mixed
speoies, the relationships beoome muoh more oompli~ted. However, it is
assumed that oaloulations of this sort give a meas~re of enhanoed weed eGOtrol
due to a broadening of the speotrum of susoeptible weeds. True
synergism, unless very pronouncsd , w::luld tend to be masked when oombinations
are evaluated against mixed speoies.
RESUIJrSANDDISCUSSION
The results are presented in Tables 1 to 5. There e.re three instanoes
where certain combinations appeared to provide oontrol of given speoies
greater than expeoted I' dimethyl tetrachloroterephthalate plus CDECagainst
barnyard grass and dimethyl tetraohlorotcrephthalate plus NPAagainst
pi~eed (Ta?le 1) ~~. ~im~~hyl_tetrachl?roterephthalate plus ohlordane