To refine selection methods for a perennial ryegrass
(Lolium perenne L.) breeding program, half-sib families
and commercial cultivars were evaluated for 3 years with treatments sown as
both single-drill rows or swards. Dry matter yield of the perennial ryegrass
treatments was evaluated several times in each year as a visual score which
was subsequently calibrated against a regression determined by cutting a
subset of plots or by cutting all plots. Thus, the experiment evaluated 2
aspects of herbage-yield determination in a perennial ryegrass breeding
program: (i) the use of visual estimates of herbage yield to reduce the time
spent cutting plots, and (ii) the use of single-row plots compared with
swards. The correlation (either as Pearsons correlation coefficient, or
Spearmans rank correlation coefficient) between visual estimates of herbage
yield was always significant (P<0.01), with the
exception of the rank correlation for sward plots in the summer 1995
(r = 0.4; P<0.05).
However, the extent of the correlation varied (r
= 0.4–0.9), and at some harvests calibrated visual ratings only
explained a small proportion of the variance observed in harvested dry matter
yields. These data suggest that visual ratings of herbage yield would be
accurate enough to be used to detect large differences between families,
breeding lines, cultivars or accessions of perennial ryegrass. However, when
differences between lines are likely to be small, then harvesting all plots
would give a more accurate estimate of the yield of perennial ryegrass lines.
Likewise, the herbage yield of perennial ryegrass in single-row plots was
significantly correlated with the herbage yield of perennial ryegrass sown as
swards (P<0.01 or P<0.05).
However, the correlation was again variable leading to the conclusion that
evaluation of perennial ryegrass as single-row plots was not always an
accurate indicator of sward yield. For those 4 (of 13) harvests over 3 years
where the interaction between sward yield and row yield of the perennial
ryegrass lines was significant (P<0.05), this
interaction was shown not to be due to significant rank changes but rather to
an increase in the differences of yield in swards or yield in single-row
plots. We conclude that the harvesting of swards was the most reliable method
of estimating the dry matter yield of perennial ryegrass cultivars. However,
significant correlations between visual rating of treatments, or yield in
single-row plots and measured yield as swards illustrated that these methods
(visual ratings and single-plot yields) could be used to reduce the cost of
evaluating differences in the herbage yield potential of perennial ryegrass,
especially when these differences were likely to be large or when seed is
limited, such as during the evaluation of accessions.
Identifying potential heterosis in perennial ryegrass
