Seed production of subterranean clover
(Trifolium subterraneum L.) in mixtures with lovegrass
(Eragrostis curvula (Schrader) Nees cv. Consol),
cocksfoot (Dactylis glomerata L. cv. Currie), phalaris
(Phalaris aquatica L. cv. Sirolan), danthonia
(Austrodanthonia richardsonii (Cashm.) H.P. Linder, cv.
Taranna), and lucerne (Medicago sativa L. cv. Aquarius)
was compared with pure and degraded (invaded by annual volunteers) annual
subterranean clover pasture at 2 sites (Junee and Kamarah) in the southern
wheatbelt of New South Wales. Seed yields, clover seedlings in winter, and the
change in the proportion of 3 subterranean clover cultivars (Dalkeith, Seaton
Park, Goulburn) when grown with and without perennials were assessed. The
effect of thinning the perennials to 10 plants/m2 on
clover seed set was examined at the drier site.
Seed production of subterranean clover in the mixtures was depressed by up to
50% compared with the pure and degraded annual swards. Initial clover
seed poduction in the mixtures was at least 60 kg/ha even in the drought
year at the wetter site (Junee), and >85 kg/ha at Kamarah, the drier
site (seedling establishment at Kamarah failed in the drought year). Clover
seed reserves in the following 2 years progressively increased to >300
kg/ha in the perennial swards at Junee but were <100 kg/ha by the
end of the third year at Kamarah. In comparison, seed reserves in the pure
clover and degraded annual swards were >650 kg/ha at Junee and >350
kg/ha at Kamarah. Reducing perennial density to 10
plants/m2 at the drier site increased clover seed
yield about 3-fold in the first year compared with unthinned perennial swards.
The increased seed yield was due to increased numbers of burrs set and
increased seeds per burr and, in all perennial pasture treatments except
lucerne, increased seed size.
Clover seedling regeneration in 3rd and 4th year after sowing was
substantially lower in the perennial-based mixtures than annual plots, with a
significant (P < 0.05) positive correlation at both
sites between clover seedling regeneration and seed bank size (1996,
r2 = 0.46–0.64; 1997,
r2 = 0.64–0.85).
Following false breaks in early autumn, clover seedling populations were
substantially higher in the pure and degraded clover treatments than in most
perennial treatments.
The proportion of the 3 cultivars present in the seed bank at the end of the
pasture phase differed between sites but the sward type only influenced the
proportion at the drier site. At the medium rainfall site, the later maturing
cultivar Goulburn constituted 27–54% of the seed bank and the
early flowering Dalkeith 25–46%, with unsown cultivars being
insignificant ( <1%). At the low rainfall site, Dalkeith was the
major component (33–52%) of the seed bank but the background
population of unsown cultivars constituted 11–48%, the lowest
proportion being in swards without a perennial component. The proportion of
Goulburn was highest (23%) in the pure sward and lowest (10%) in
lucerne and phalaris.
It was concluded that subterranean clover could form relatively stable
mixtures with perennials in medium rainfall environments, with clover
populations increasing with time. In lower rainfall environments, clover
seedling populations in perennial swards may be low due to reduced seed set
and decreased seedling survival following early autumn rains. In these
environments earlier maturing, hard-seeded cultivars are more likely to
persist in mixtures and there is more potential for unsown cultivars to
constitute a greater proportion of the sward. Decreasing perennial density
offers scope for improving clover seed set and survival in these environments.