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ANNUAL MEDIC CULTIVAR MIXTURES IN SEMI-ARID FARMING SYSTEMS

R.A. Latta1 and E.D. Carter2

1Victorian Institute for Dryland Agriculture, Mallee Research Station, Walpeup, Vic 3507
2
Roseworthy College, The University of Adelaide, Roseworthy, SA 5371

Summary. This paper compares the effects of chemical grass control and stocking rate treatments imposed on two medic cultivars of differing maturity (Harbinger AR and Paraggio) during plant development and seed maturation period. The cultivars relative seed production and seed persistence (longevity) were measured over a three year pasture-pasture-wheat rotation.

INTRODUCTION

The development of annual medic cultivars adapted to the semi-arid regions of south-eastern Australia has been an important component in the evolution of the ley farming system. Bluegreen aphid (Acyrthosiphon kondoi Shinji) resistant lines Medicago truncatula cv. Paraggio and M. littoralis cv. Harbinger AR were first recommended for sowing in the Victorian Mallee in 1984 and 1991 respectively.

Paraggio has inherently higher percentages of water permeable seed (up to 30%) at the seasonal break than the previously recommended cultivars, M. truncatula cv. Jemalong and M. littoralis cv. Harbinger (approximately 10%). This causes increased seed losses as a result of summer rains (5, 8). Harbinger AR was considered agronomically similar to Harbinger (10). Cultivars with less hard-seedeedness at the seasonal break increase the plant density and thus the competitiveness and productivity of the medic-based pasture (6). The most persistent medic cultivars for the semi-arid zones of south-eastern Australia flower during late winter (3, 4). Harbinger flowered after 81 days and Paraggio after 114 days at Walpeup, following a mid-April sowing (7).

Harbinger AR has significantly smaller seed than Paraggio (10). Carter et al. (2)measured a direct relationship between seed size and survival after ingestion with small, hard-seeded pasture legumes better suited for survival in self-regenerating stocked pastures.

Medic seed reserves of 200 kg/ha following a cropping sequence was considered the critical minimum seed reserve required to maintain the stability of self-generating medic pastures in the ley farming system (1).

This paper describes an experiment to establish the persistence and productivity of two agronomically diverse medic cultivars when treated with grass selective and broad-spectrum herbicides combined with grazing by sheep .

MATERIALS AND METHODS

The experiment was conducted at the Mallee Research Station, Walpeup, Victoria, (Lat. 35.80 S, Long. 1420 E, Alt. 50 m). The soil was calcareous earth (9) with pH in top 10 cm being 7.7 (H2O). A mixture of Medicago truncatula cv. Paraggio (4 kg/ha) and Medicago littoralis cv. Harbinger AR (3 kg/ha) was sown on 22 April 1991. The 10 ha site was fenced into paddocks for grazing with sheep at high (Hsr) and low (Lsr) stocking rates over the 2-year pasture phase of the experiment (1991 and 1992).

The stocking rates were:

(i) 1 DSE/ha in 1991, 2.5 DSE/ha in 1992 (Lsr).

(ii) 2 DSE/ha in 1991, 5 DSE/ha in 1992 (Hsr).

Three pasture herbicide treatments were applied in 1991, one further herbicide treatment was applied in 1992. The stocking rate paddocks were replicated twice, the herbicide treatments were randomised twice within each paddock making four replicates.

The herbicide treatments were as follows:

• Pasture topping with 330 ml/ha of glyphosate, 360 g a.i./L, at grass anthesis on 26 September 1991, to inhibit seed set (Pt).

• Winter cleaning with 330 ml/ha of glyphosate, 360 g a.i./L, at the 6 to 8 leaf stage of the medic on 31 Ju1y 1991, to restrict annual grass development (Wc).

• Selective grass control with 500 ml/ha of Fusilade (fluazifop, 212 g a.i./L), at the 6 to 8 leaf stage of the medic on 1 August 1991, to eradicate annual grasses (Sel).

• Winter cleaning with 330 ml/ha of glyphosate, 360 g a.i./L, at the 6-8 leaf stage of the medic on 2 Ju1y 1992, to restrict annual grass development (Wc92).

Wheat (cv. Meering) was sown at 50 kg/ha with 50 kg/ha of Pivot Double Super + Zinc (15.8% phosphorus, 4% sulphur and 2.5% zinc) on 1 July 1993 into a moist seed-bed. Fertiliser was not applied during 1991 or 1992.

In December 1991 and 1992, the effects of the stocking and herbicide treatments on the medic seed yield was measured by collecting seed pods from each plot, separated into species and threshing, cleaning and weighing seed. In July 1993 and February 1994, prior to the opening seasonal rains, the medic seed reserves present were measured from soil cores (0-5 cm) taken from each plot. The seedpods were removed from the soil and the species separation made in a 5.6 mm sieve. The seedpods were threshed and the seed cleaned and weighed.

RESULTS AND DISCUSSION

The seasonal rainfall over the course of the study contrasted markedly. In 1991 there was a dry autumn and spring but a wet winter (Table 1). In 1992 there was a wet autumn and spring but a drier period during winter. In 1993 there was a dry autumn but a wet spring.

Table 1. Growing season rainfall (mm) for 1991, 1992, 1993 and the long-term mean (1911-1993).

 

Apr.

May

June

July

Aug.

Sep.

Oct.

Annual Total

1991

18

0

68

54

32

26

0

303

1992

43

82

23

15

44

88

53

520

1993

1

6

13

51

22

78

44

361

Long-term mean

21

33

31

32

35

33

36

337

Differences in seed yield occurred between the 2 pasture years due to differences in growing season rain (170 mm 1991; 300 mm 1992).

Table 2. The effect of grazing and herbicide treatments applied during 1991 and 1992 on seed yields (kg/ha) of Harbinger AR and Paraggio.

Stocking

Herbicide

Seed yield

rate

treatment

1991

1992

   

Harbinger AR

Paraggio

Harbinger AR

Paraggio

 

Pt

10

14

119

214

Lsr

Wc

57

92

138

255

 

Sel

122

77

222

333

 

Wc92

71

48

135

182

 

Pt

22

14

109

181

Hsr

Wc

65

46

176

219

 

Sel

92

53

264

217

 

Wc 92

70

41

146

120

l.s.d.(P=0.05)

43.8

62.4

The 1991 high stocking rate (Hsr) did not reduce seed yields. Harbinger AR had higher seed yields than Paraggio when glyphosate was not used in 1991. However, the Paraggio seed yield tended to be lower as a result of the Hsr. In 1992, Paraggio produced more seed than Harbinger AR at the Lsr. However, the Hsr reduced the seed yield of Paraggio. The selective grass control (Sel) produced similar or more seed and pasture topping (Pt) generally reduced seed yields in both years irrespective of cultivar or stocking rate (Table 2).

Table 3. Seed reserves (kg/ha) of Harbinger AR and Paraggio prior to the seasonal break in 1993 and 1994 in response to grazing and herbicide treatments applied in 1991 and 1992.

Stocking

Herbicide

Seed reserves

rate

treatment

1993

1994

   

Harbinger AR

Paraggio

Harbinger AR

Paraggio

 

Pt

171

213

93

180

Lsr

Wc

198

249

81

202

 

Sel

275

311

123

204

 

Wc92

150

239

48

162

 

Pt

113

186

81

154

Hsr

Wc

215

176

96

131

 

Sel

316

226

114

141

 

Wc92

201

152

62

74

l.s.d.(P=0.05)

108.3

62.4

In 1993 Harbinger AR at Lsr tended to have lower seed reserves than Paraggio after the first 2 years of the pasture-pasture-cereal-pasture rotation (Table 3). However, with the Hsr imposed seed reserves were comparable. The Harbinger AR seed reserve reduced at a greater rate in comparison to Paraggio during the 1993/94 crop year. This may have been due to high mouse numbers, and their ability to extract seeds from the Harbinger AR seedpod relative to the spinier Paraggio seedpod (Latta, observations).

With a growing season of four months in 1991 and below average rain, Harbinger AR seed yields were generally higher than Paraggio. The increased seed yield of Harbinger AR compared to Paraggio was not reduced by the higher stocking rate. In 1992, Paraggio produced a higher seed yield than Harbinger AR under both low and high stocking rates. Paraggio better utilised the longer, seven month growing season of 1992 and above average rain. A total seed reserve of approximately 250 kg/ha (Paraggio and Harbinger AR seed) was available to regenerate in the 1994 pasture phase. Assuming a 50% establishment rate, a density of at least 350 medic plants/m2 should ensure.

CONCLUSIONS

Findings of consequence to semi-arid ley farming systems include:

• The value of mixing cultivars with differing maturity and agronomic characteristics, an early maturing small-seeded cultivar (Harbinger AR) with a mid-season larger-soft-seeded (Paraggio), to maximise seed production and subsequent regeneration over a range of seasons has been established.

• Smaller-seeded cultivars help ensure seed persistence due to seed yield and seed reserve reduction in larger-seeded cultivars when stocking rates are increased.

• The reduction of the medic seed yield as a result of the non-selective herbicide applied at grass anthesis; the inherent tolerance of Paraggio to the glyphosate applied as a winter cleaning treatment.

• Medic pastures can be established and maintained with a low grass component at plant densities of about 400 plants/m2. This will support a substantial increase in farm productivity through an increased legume component, resulting in more livestock, greater nitrogen accumulation, decreased root diseases, notably Take-all and Cereal cyst nematode leading to higher livestock and cereal returns.

ACKNOWLEDGMENTS

The Grains Research and Development Corporation generously provided financial support.

REFERENCES

1. Carter, E.D. 1981. Proc. XIV Int. Grassl. Congress Lexington, Ky, U.S.A. June 1981. pp. 447-450.

2. Carter, E.D., Challis, S. and Knowles, R.C. 1988. Workshop on Tillage Systems, Rotations, Nitrogen and cereal Root Diseases. Adelaide, South Australia. p. N2.

3. Cornish, P.S., 1985a. In: The ecology and agronomy of annual medics. (Ed Z. Hochman). NSW Dept. Agric. Tech. Bull. 32. pp. 13-16.

4. Cornish, P.S., 1985b. In: The ecology and agronomy of annual medics. (Ed Z. Hochman). NSW Dept. Agric. Tech. Bull. 32. pp. 17-22.8

5. Crawford, E.J., Lake, A.W. H. and Boyce, K.J. 1989. Adv. in Agronomy. 42, 399-436.

6. Crawford, E.J., and Nankivell, B.J. 1989. Aust. J. Exp. Agric. 29, 183-188.

7. Hochman, Z. 1987. Aust. J. Agric. Res. 38, 279-286.

8. Latta, R.A., and Quigley, P.E. 1993. Aust. J. Exp. Agric. 33, 443-449.

9. Northcote, K.H. 1979. A Factual Key for the Recognition of Australian Soils. 4th Edition. (Rellim Tech. Publ: Glenside, SA).

10. Oram, R.N. 1990. In: Register of Australian Herbage Plant Cultivars. CSIRO p. 221.

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