The yield of a romaine lettuce field that is harvested for full size
heads depends on the number of lettuce plants per ha that are
marketable based on a minimum head size, and other quality characteristics
(i.e. disease symptoms, physiological defects, and insect
damage). Assuming that head size was the only criteria limiting
marketable yield, monoculture lettuce (L100) at a density of
65,333 lettuce plants per ha has a potential maximum yield of
2722 boxes per ha of full size heads (24 heads per box). Based on
the lettuce plant densities of the various intercropped treatments
in this study (Table 1), the yield potential in 24 count boxes per
ha in the replacement treatments would theoretically range from
2500 boxes (in A100 where alyssum displaced 8% of the lettuce)
to 2677 boxes (in L75A25 where alyssum displaced 2% of the lettuce),
and 2722 boxes in the additive treatments where alyssum
was added to the standard density of L100. Although this suggests
that there would be a yield advantage in the additive versus
replacement treatments, additional research is needed to determine
if the lettuce plants in the additive treatments that produced
approximately 25% less shoot DM than L100, are able to produce
large enough heads for marketable size standards. The reduction
in lettuce shoot dry DM that was apparent in the additive treatments
and all replacement treatments except L50A50s, may have
a relatively minimal effect on the weight of a box of lettuce or
the total yield of a field after the harvest crew trims down the
heads and packs them in boxes. The gross weight of a 24 count box
of organic romaine lettuce for wholesale in this region can range
from approximately 11–14 kg per box, however, a minimum of
13.6 kg per box may be necessary for wholesale during a typical year
when the lettuce supply is high (Brennan, unpublished data). To
achieve a 13.6 kg box weight for 24 heads, the average trimmed
fresh head weight would need to be approximately 515 g which corresponds
to approximately 31 g of DM; this assumes that the empty
box weight is 1.24 kg and a typical moisture content of 94%. The
average DM of the untrimmed heads in this study ranged from 38
to 51 g (Fig. 1). It is important to point out that during commercial
lettuce harvests, individual heads are not weighed, however, quality
control inspectors routinely check the weight of a random sample of
packed boxes. Furthermore, the box weight requirements are somewhat
flexible, particularly with organic lettuce, depending on the
lettuce supply and demand at various times of the year.