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Trade-offs for ratooning sorghum after harvest to provide forage for grazing

Jeremy Whish and Lindsay Bell

CSIRO Sustainable Ecosystems, 203 Tor St, Toowoomba, Qld 4352 Email jeremy.whish@csiro.au

Abstract

Allowing a sorghum crop to re-grow after harvest can provide valuable forage for livestock, but what is the cost to following crops? Will the plant available water (PAW) used by regrowing sorghum significantly reduce the starting water for a following crop, resulting in reduced grain yield? A simulation analysis using the APSIM-grain sorghum module and a modified forage sorghum-module was used to assess the implications of sorghum regrowth on the plant available water and grain yield of the following crop. Allowing an October sown sorghum crop to regrow reduced the average PAW at the following October sowing by 38, 34, 30, and 17 mm which translated to an average sorghum yield loss compared to a harvest terminated sorghum crop of 0.38, 0.44, 0.47 and 0.30 t/ha for Roma, Nindigully, Goondiwindi and Warialda respectively. However, the reduction in PAW was less (3-18 mm) after a longer fallow to wheat, which is common practice in the region. The cost in terms of water and yield for allowing sorghum to regrow were reduced along a transect from west to east due to the cooler climate and increased winter rainfall.

Introduction

A highly variable summer dominant rainfall, with high evaporation and rainfall intensity, characterise the northern grains region (Felton et al. 1995). Sorghum regrowth can provide high quality forage to grow livestock during the autumn when the quality of grasses is declining and before winter forage is available for grazing. The decision to allow sorghum crops to regrow after harvest has initiated the question, will the water used by the regrowing crop be replaced before the next crop or will it reduce the available stored water and result in a yield loss? Water is the underlying driver of northern farming systems (Waring et al. 1956, Whish et al. 2005, Whish et al. 2007b) and the decision to use water for a short-term benefit of additional biomass has to be weighed against the impact on yield of the next crop. This paper uses the APSIM simulation model (Keating et al. 2003) to compare the soil water status after a sorghum crop sprayed out at harvest to one that is allowed to re-grow until 1 May.

Methods

The APSIM simulation model was used to predict the depletion of soil water by sorghum re-growth using historical meteorological records from 1957 to 2007 for 4 locations across the region (Warialda, Nindigully, Roma, and Goondiwindi). The current grain sorghum module in APSIM does not allow grain sorghum to re-grow after harvest. A simplified grain sorghum module based on forage sorghum was developed and parameterised to produce similar biomass and water use results to grain sorghum (Fig. 1). At this stage the water use in this simplified sorghum module, is slightly higher than that of the grain sorghum. This simplified module was used in combination with the grain sorghum module to assess the impact of allowing crops to re-grow after harvest. All simulations were performed on a Grey Vertosol soil with a plant available water-holding capacity (PAWC) for sorghum of 220 mm. Simulations were run with sorghum sown on 15 October at a density of 70 000 plants per ha on a 1 m row spacing. Sorghum crops allowed to regrow after harvest were terminated on the 1 May coinciding with the average time for the first frost. Sorghum crops were fertilised by the addition of 150 kg/ha of urea at sowing to minimise N limitations on grain yield; no additional nitrogen was applied to the sorghum re-growth.

Figure 1: Modelled biomass and plant available water (PAW) predictions of the existing APSIM-sorghum module compared to the modified APSIM-forage sorghum module.

Results

After the sorghum was allowed to regrow compared to a crop killed at harvest, the soil PAW was on average between 33 and 42 mm lower on the 15 May for the 4 locations. In some years (between 4-12 % of years) quite large differences in PAW were observed (>100 mm, Table 1) whereas minimal differences (<20 mm) occurred in 41, 31, 29 and 23 % of years at Warialda, Goondiwindi, Nindigully and Roma respectively. At the more western sites the difference in PAW was larger and persisted longer even until the following May. However, for all sites the magnitude of the difference in PAW reduced over time. Warialda, the most eastern site with a lower average temperature and higher and more reliable winter rainfall, was least affected by the re-growing crop. Western soils were regularly affected by the re-growing sorghum with a PAW deficit difference for sorghum on sorghum of (<40mm) occurring 52 and 33% of the time in Roma and Nindigully respectively.

Table 1. The difference in plant available water (PAW) between sorghum killed at harvest and sorghum allowed to re-grow until 1 May. The impact of the re-growing sorghum crop on PAW was more pronounced for the western and north-western sites of Nindigully and Roma.

Date after sorghum crop (Likely crop option)

Sites

Average PAW deficit (mm)

Percentage of years that the difference in PAW deficit is

 

>20 mm

>40 mm

>100 mm

15 May (Wheat double crop sorghum )

Warialda
Goondiwindi Nindigully
Roma

33
36
39
42

59
69
71
77

30
38
47
55

8
4
8
12

15 Oct (Early sorghum, sorghum-sorghum)

Warialda
Goondiwindi Nindigully
Roma

17
30
34
38

39
52
64
73

16
29
33
52

2
2
8
8

15 May (Wheat after long-fallow, Sorghum long fallow-wheat)

Warialda
Goondiwindi Nindigully
Roma

3
17
18
14

8
33
33
35

0
14
19
16

0
2
0
0

The effect of ratooning a sorghum crop on the yield of a following sorghum crop reflected the difference in PAW at sowing (Table 2). On average the yield deficit was 0.3-0.5 t/ha after a ratooned sorghum crop compared to a previous crop sprayed out at harvest. Nonetheless, in about 40% of years little impact on grain yield would occur. At Warialda the effect was least, with little impact on grain yield (<0.2 t/ha) in more than half the years. The severity and frequency of loss were somewhat higher at the western sites of Nindigully and Roma (Table 2).

Table 2. The reduction in grain yield for an October-planted sorghum crop after a ratooned sorghum crop compared to when a preceding sorghum crop was killed at harvest for 4 sites in the northern cropping region .

Location

Average Sprayed sorghum yield (t/ha)

Average yield deficit (t/ha)

Percentage of years that the yield deficit is (t/ha)

 

< 0.2

0.2-0.5

0.5-1.0

>1.0

Warialda

3.8

0.30

50

25

21

4

Goondiwindi

2.8

0.47

46

17

23

14

Nindigully

2.3

0.44

38

31

19

12

Roma

3.0

0.38

38

29

23

10

Discussion

PAW at sowing is an important component of crop production in the northern grains region due to the highly variable rainfall (Waring et al. 1956, Whish et al. 2007a,b). In this farming system, water is a valuable finite resource that has to be partitioned between enterprises and crops. The decision on how to partition the water is difficult and will depend on the priorities and motivations of an individual farmer.

The results from this work highlight the cost of sorghum regrowth on following crops. This cost could be reduced by strategically fitting the practice of allowing sorghum to regrow within the system, e.g. by allowing regrowth only on paddocks that are to be long fallowed into wheat allowing time for rainfall to replenish the soil PAW, which in turn will reduce the impact on the wheat yield.

This work has focussed on the cost of sorghum regrowth to the supply of soil water for following crops. There is also likely to be some impact on nitrogen availability within the system, especially on old cultivation paddocks.

On the other hand, the extra forage produces potential benefits in livestock production. The benefits from this additional forage have not yet been economically compared, since the simulated sorghum regrowth is yet to be field tested. Further work to quantify potential livestock production benefits and potential costs are required to fully understand the tradeoffs involved in allowing sorghum to regrow.

References

Felton WL, Marcellos H, Martin RJ (1995). A comparison of three fallow management strategies for the long-term productivity of wheat in northern New South Wales. Australian Journal of Experimental Agriculture 7, 915-921.

Keating BA, Carberry PS, Hammer GL, Probert ME, Robertson MJ, Holzworth D, Huth NI, Hargreaves JNG, Meinke H, Hochman Z, McLean G, Verburg K, Snow V, Dimes JP, Silburn M, Wang E, Brown S, Bristow KL, Asseng S, Chapman S, McCown RL, Freebairn DM, Smith CJ (2003) An overview of APSIM, a model designed for farming systems simulation. European Journal of Agronomy 18, 267-288

Waring SA, Fox WE, Teakle LJH (1958). Fertility investigations on the Black earth wheatlands of the Darling Downs, Queensland. Australian Journal of Agricultural Research 9, 205-216.

Whish JPM, Castor P, Carberry PS, Peak A, (2007) On-farm assessment of constraints to chickpea (Cicer arietinum L.) production in marginal areas of northern Australia. Experimental Agriculture 37, 505-512

Whish JPM, Castor P, Carberry PS (2007) Managing production constraints to the reliability of chickpea (Cicer arietinum L.) within marginal areas of the northern grains region of Australian Journal of Agricultural Research, 58, 396-–405

Whish JPM, Butler G, Castor M, Cawthray S, Broad I, Carberry P, Hammer G, McLean G, Routley R, and Yeates S (2005) Modelling the effects of row configuration on sorghum yield reliability in NE Australia. Australian Journal of Agricultural Research 56, 11-23.

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