Recently Campbell et al.
(2011) in describing breeding progress with Pee Dee germplasm in the US shows increasing yield (28 kg/ha per breeding group).
However length and strength were reduced, so the negative association between yield and length and strength was retained.
The slopes we have calculated from Campbell et al. (2011) data were −0.0045 mm/kg/ha for length and −0.076 kN m kg−1/kg/ha for strength,
similar to the US data we calculate here (Table 2 and Fig. 1 and Fig. 2).