Vol. 51—1997 - NorthEastern Weed Science Society

Waggoner December 24, 1994
141
The next joint is weeds to yield. Cousens (1985) concluded
that the model represented by the curve in Fig 9 excelled in
fitting observ tions of weeds and yield. The yield of Norwin
wheat grown by Blackman (1994) in 1990-91 declined as
. ( w)
X W+ W2!
where weeds w an decrease yield at most by the fraction ~x,
and w2 decreas s yield by half ~x.
Fig 9. The dec ining yield of Norwin wheat in 1990-91 as the
density of the weed, downy brame, increased. (Blackman 1994).
10'~'----+---------------"
80
20
0'-----...,1----------------"
o 50
100
Weeds/sq
m
150
200
Ortiz-Monaste io (1994) applied nitrogen fertilizer to wheat,
and a model r sembling that of the harm of weeds fits the
benefit of fe ti1izer. The yield of weed-free wheat equals
where Yx is t e maximum yield of weed-free crop. In kg/ha, n
is nitrogen a ded by fertilizer, ns that from the soil and n2
fertilizer n hat produces a yield of half Yx' I assume that