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Gibberellic acid and early dry matter production of dual purpose wheat

Geoff Dean1 and Tina Botwright Acua2

1 Tasmanian Institute of Agricultural Research, Mt Pleasant Research Laboratories, PO Box 46, Kings Meadows, Tas 7249. www.tiar.tas.edu.au Email Geoff.Dean@dpiw.tas.gov.au
2
Tasmanian Institute of Agricultural Research, The University of Tasmania, Private Bag 98, Hobart, Tas 7001. www.tiar.tas.edu.au Email Tina.Acuna@utas.edu.au

Abstract

The use of gibberellic acid (GA) to boost pasture production over winter has recently received considerable attention. There have been no scientific reports of GA being used to increase dry matter production in dual purpose cereals for grazing and grain production. A trial was conducted to assess the effect of GA applied at three rates (4, 8, 16 g/ha) on dry matter production and subsequent grain yield of dual purpose wheat (cultivar Tennant) grown in northern Tasmania. There were large and significant increases in dry matter (DM) production with application of GA. The highest rate resulted in a more rapid increase in DM but by week four all GA treatments had produced significantly higher DM than untreated wheat. Mean DM increases from GA treatment were around 30%. In contrast, after a final cut the untreated wheat produced significantly more DM than all the GA treatments suggesting treated plants lacked sufficient reserves for re-growth. Application of a foliar nutrient spray (N:P:K, 9:4:6) did not significantly improve DM production or plant recovery in GA treatments. The reduction in plant DM did not significantly affect grain yield but competition from ryegrass may have confounded the yield data. Application of GA may be a useful management tool to provide increased DM production over a critical period of feed shortage in winter. The reduction in plant growth over spring is of less consequence as there is generally sufficient feed over this time period.

Key Words

applied gibberellin, applied foliar NPK

Introduction

The plant growth regulator, gibberellic acid (GA), is well-documented to increase stem elongation and leaf blade length when applied to GA-sensitive cereals during vegetative growth (e.g. Evans et al., 1995). The growth-promoting ability of gibberellic acid (GA) has been exploited in pasture species such as ryegrass, lucerne and kikuyu to increase dry matter (DM) production (Bidlack and Buxton, 1995), particularly during the winter feed gap (Arnold, 1967; Percival, 1980). However, regrowth and forage quality of pasture grasses are reportedly reduced after cutting (Percival, 1980; Bidlack and Buxton, 1995). There have been no scientific reports of GA being used to increase DM production in dual purpose cereals. A trial was conducted to quantify the effects of GA with and without a foliar nutrient spray, consisting of N:P:K (9:4:6 plus copper and zinc), on DM production of dual purpose wheat and to assess the effects on subsequent plant recovery and grain yield.

Methods

The winter wheat cultivar, Tennant, was sown at 90 kg/ha with 200 kg/ha of N:P:K fertiliser (14:16:11) at Cressy in northern Tasmania on 10 April 2007. The recommended rate of GA for application to pastures is up to 8 g/ha. Applied commercially as ProGibb (400 g GA/kg) this equates to 20 g/ha (Valent Biosciences, 2006). GA was applied at rates of 0 (nil), 4, 8 and 16 g/ha. Two additional treatments of GA (nil and 8 g/ha) received a foliar nutrient spray, consisting of 20 L/ha of N:P:K (9:4:6 plus copper and zinc). Plots were 10 m long x 2 m wide with four replicates in a randomised complete block design. Plots received an initial grazing and treatments were then applied at mid tillering on 20 July or 102 days after sowing (DAS). Plant dry matter at the time of GA treatment was 0.48 t/ha. Dry matter (DM) cuts (3 x 0.25 m quadrats) to ground level were taken at weekly intervals for four weeks until mid August. All plots were mown on 17 August and 3 September (130 and 147 DAS, respectively) to simulate grazing. Feed quality of samples from the 17 August cut was measured at FeedTest Laboratories, Hamilton. A final DM cut was taken 15 d later on 18 September (162 DAS) to assess subsequent recovery. Plots were machine-harvested for grain yield on 23 January 2008.

Results

Before mowing, DM production increased over time with applied GA (Figure 1). For the first two harvests at 109 and 116 DAS, only GA applied at the highest rate of 16 g/ha produced significantly more DM than the control. By the third harvest at 123 DAS, all plants with applied GA produced more DM than the control, irrespective of rate. Mean DM increases were around 30%, from 1.26 t DM/ha in control plots to a mean of 1.64 t/ha in GA treated plots. There was little change in DM production in either control or GA treated plants from the third to fourth harvest, at 130 DAS. After mowing, untreated wheat showed significantly better recovery. There was no change in DM production when the NPK foliar spray was applied, either alone or in combination with GA applied at the rate of 8 g/ha (Table 1).

Leaves of GA treated plants were paler in colour and this was incremental with rate (Table 1). Previous studies have reported reduced chlorophyll in GA treated pasture plants (e.g. Williams and Arnold, 1964). Application of the foliar spray improved plant colour in GA treated plants (Table 1). Feed quality of pooled replicates tended to decline when GA was applied at 8 g/ha, with crude protein around 2% lower and fibre (NDF) 1-2% higher than the control (data not shown).

Plant height at maturity was 3 cm shorter when GA was applied at rates of greater than 8 g/ha, regardless of foliar spray (Table 1). This effect has implications for reduced crop lodging and stubble management of subsequent crops. There was no consistent effect of applied GA or foliar spray on grain yield (Table 1). Competition with ryegrass plants was likely to have contributed to a lack of significant differences in grain yield between treatments. Estimates of percentage ryegrass in samples cut from plots ranged from 5-11% but this was variable and did not relate to treatment.

Figure 1. Dry matter production of Tennant wheat over time after application of four rates of GA (g/ha), Cressy, Tasmania 2007-08. *The crop was mown at 130 and 147 DAS. Bars represent the lsd at P = 0.05.

Table 1. Dry matter production (DM), visual leaf colour score (0, yellow; 5, green), plant height and grain yield of Tennant wheat following application of different rates of GA and a foliar nutrient application, Cressy, Tasmania 2007-08. *The crop was mown at 130 and 147 DAS.

GA

Foliar spray

DM (t/ha)

Colour score

Plant height

Grain yield

(g/ha)

(l/ha)

109 DAS

116 DAS

123 DAS

130 DAS

162 *DAS

(0-5)

(cm)

(t/ha)

0

0

0.50

0.80

1.24

1.26

1.19

5.00

96.6

4.49

4

0

-

0.88

-

1.59

1.03

4.00

95.8

4.02

8

0

0.59

0.92

1.54

1.65

0.98

3.25

93.4

4.31

16

0

0.66

1.08

1.62

1.68

0.93

3.50

92.9

3.85

0

20

0.52

0.95

1.37

1.31

1.15

5.00

95.7

4.10

8

20

-

1.05

1.48

1.74

0.85

4.00

92.6

3.91

                   

lsd (P=0.05)

0.10

0.19

0.22

0.22

0.12

0.40

2.5

n.s.

Conclusion

Application of GA increased dry matter production of the winter wheat cultivar, Tennant. Crop regrowth following cutting and possibly feed quality were reduced in GA treated plants, which were shorter than untreated plants. Total DM production over time, however, was greater in GA treated plants compared with the control. These results are consistent with pasture grasses as reported by Percival (1980) and Bidlack and Buxton (1995). While GA-treated plants apparently lacked sufficient reserves after cutting to promote an equivalent rate of regrowth compared with the control, total DM production over time was 14% larger and grain yield tended to be slightly less, but not significantly different, than control plants. Importantly, additional feed is produced at a critical time when pasture growth is slow. The application of a foliar nutrient spray in combination with GA did not alleviate the reduction in DM production after cutting. Application of GA to dual purpose winter wheat during vegetative growth may be a viable management strategy for growers to ensure adequate stock feed during the winter feed gap. A small reduction in regrowth after grazing should be acceptable, if other feed sources are available and the impact on grain yield minimal. Further field trials are in progress to confirm our findings, in particular the effect of applied GA on grain yield.

Acknowledgements

This research was supported by the Grains Research and Development Corporation. The authors thank Simon Munford and Brett Davey for technical support and Mike, Will and Tom Green for provision of land for the trial.

References

Arnold GW, Bennett D and Williams CN (1967). The promotion of winter growth in pastures through growth substances and photoperiod. Australian Journal of Agricultural Research 18, 245-257.

Bidlack JE and Buxton DR (1995). Chemical regulation of growth, yield and digestibility of alfalfa and smooth bromegrass. Journal of Plant Growth Regulation 14, 1-7.

Evans LT, Blundell C and King RW (1995). Developmental responses by tall and dwarf isogenic lines of spring wheat to applied gibberellins. Australian Journal of Plant Physiology 22, 365-371.

Percival NS (1980). Cool-season growth responses of Kikuyu grass and ryegrass to gibberellic acid. New Zealand Journal of Agricultural Research 23, 97-102.

Valent Biosciences (2006). ProGibb SG, Plant Growth Regulator.

Williams CN and Arnold GW (1964). Winter growth stimulation by gibberellin in differentially grazed pastures of Phalaris tuberosa. Australian Journal of Experimental Agriculture and Animal Husbandry 4, 225-230.

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