Cuphea Production and Management - Industrial Crops: Breeding for BioEnergy and Bioproducts - page 301

with N, P, and K, prior to planting at 56, 56, and 22 kg ha 1 of each nutrient
respectively [ 6 ] based on soil test results [ 8 ].
A recent study conducted in North Dakota and Minnesota by Berti et al. [ 15 ], in
which fertility treatments (soil + fertilizer) were 44, 60, 80, 100, 150, and
200 kg N ha 1 , showed that increased N fertility enhanced N uptake and seed
yield in cuphea. According to a regression model of the response, the maximum
total N uptake at harvest was 139 kg N ha 1 and the maximum seed yield occurred
at 185 kg N ha 1 . However, the seed yield increase obtained with added fertilizer
(134 kg N ha 1 ) was only 71 kg ha 1 . Berti et al. [ 15 ] concluded that N fertilizer
application, at least for already fertile soils, may not be economic given the minimal
yield increase with increased N rate application.
Cuphea Weed Control
Cuphea grows very slowly the first 4 weeks after emergence in the spring. Thus,
cuphea is a very weak competitor with early-season weeds. However, once cuphea
reaches its reproductive phase (generally around midsummer), it grows vigorous
and fills its canopy quickly, thus, competing well against late-season weeds. Soil-
applied herbicides are primarily recommended to control broadleaved weeds.
Cuphea tolerates preplant incorporated herbicides such as trifluralin (840 g
a.i. ha 1 ) and ethalfluralin (840 g a.i. ha 1 ) and preemergence-applied herbicides
such as isoxaflutole (80 g a.i ha 1 ). For postemergence broadleaf weed control,
mesotrione (105 g a.i. ha 1 ) and imazethapyr (70 g a.i. ha) can be applied after
cuphea plants have at least three pairs of leaves showing [ 13 ]. Combinations of soil-
applied with postemergence-applied herbicides such as imazethapyr and
mesotrione also do not damage cuphea [ 13 ]. Two weed species that have become
problems in commercial fields of cuphea grown in Minnesota and North Dakota
that are not controlled by any combination of these herbicides are Canada thistle
( Cirsium arvense L.) and biennial wormwood ( Artemisia biennis Willd.). Recently,
however, Forcella et al. [ 14 ] showed that cuphea tolerates clopyralid at applied
rates of up to 400 g a.i. ha 1 without significant plant damage. Therefore, clopyralid
at a rate of 200 g ha 1 , commonly used for other commercial crops, can safely be
applied to cuphea to effectively control Canada thistle and biennial wormwood.
Since cuphea is a dicotyledonous plant, several graminicides can be used to
control grass weeds. Sethoxydim (0.3 kg ha 1 ) is routinely used in cuphea to
control grass weeds [ 6 , 77 ]. If adequate row spacing is used for production,
mechanical weed control such as cultivation is a viable option and has been used
successfully to control weeds prior to canopy closure.