Subterranean clover seedling numbers and growth in swards containing 1 of 5
perennial pasture species [phalaris
(Phalaris aquatica) cv. Sirolan, cocksfoot
(Dactylis glomerata) cv. Currie, lucerne
(Medicago sativa) cv. Aquarius, wallaby grass
(Danthonia richardsonii) cv. Taranna, and lovegrass
(Eragrostis curvula) cv. Consol] were compared with
those in typical annual pastures and pure clover swards in the wheatbelt of
eastern Australia.
Presence of a perennial species or the volunteer annual grass
(Eragrostis cilianensis) increased the rate of drying of
the soil surface (0–5 cm) after late February and May rain, compared
with subterranean clover swards. Perennials differed in the rate they dried
the soil surface, with the more summer-active lucerne and consul lovegrass
drying the profile more rapidly than phalaris. The amount of water in the
surface 5 cm, 6 days after the rainfall event on 27–28 February, was
strongly negatively correlated (r =
–0·75, P < 0·01) with the amount
of green perennial biomass, but not related to standing dead material or
surface residues.
Where perennials were present, a smaller proportion (2–4%) of the
clover seed pool produced seedlings in response to late summer rain, compared
with pure clover swards (18%). A higher proportion of the seed pool
produced seedlings (19–36%) following rain in late autumn but
there was no difference between species.
The more summer-active perennials (cocksfoot, danthonia, and lucerne) markedly
depressed the survival of emerged clover seedlings following both
germinations. Of the seedlings that emerged in early March, the proportion
remaining by 29 March was 57% in phalaris, 21% in lucerne,
13% in danthonia, and 1% in cocksfoot, compared with a
78% increase in seedlings in pure subterranean clover swards. By 15
May, all perennials had <2 clover seedlings/m2
surviving, compared with 37 in the annual pasture and 964
plants/m2 in pure subterranean clover. Following the
May germination, the highest proportion of emerged seedlings surviving until
29 May was in the phalaris swards (40%) and least in the cocksfoot and
danthonia swards (2–4%).
Presence of a perennial or annual grass decreased (P
< 0·05) relative water content of clover seedlings on 15 March from
74% in pure clover swards, to 48% in annual pasture, 34%
in phalaris, and 29% in lucerne swards. Clover seedlings growing in
pure subterranean swards on 15 March (17 days after germinating rain) were 4
times larger than those in lucerne and twice as large as those in either
phalaris or annual pasture. Seed size did not differ between treatments, but
available mineral soil nitrogen was significantly higher
(P < 0·001) in pure subterranean clover swards
(32 mg N/g) compared with perennials (3–13 mg N/g).
Strategies such as heavy grazing in late summer to reduce green biomass of the
perennials or sowing the perennials at lower densities may reduce the adverse
effects that perennials have on subterranean clover seedlings in these drier
environments.
