Chapter 2: Literature review
2.10 Sugarcane yield decline
This chapter to date has focused on issues relating to the soil resource itself. Soil is the medium in which the sugarcane plant grows. The soil and the way in which it is managed will have a major impact on sugarcane yield.
There is increasing evidence of a levelling off trend or decline in sucrose yield per hectare in many sugarcane producing countries worldwide (Sumner, 2011). South Africa has experienced a long-term yield plateau in yield production since 1971 (Meyer & van Antwerpen, 2001). Declining sugarcane yields have also been recorded in Australia, United States of America, Barbados, Colombia and Hawaii sugarcane industries (Haynes & Hamilton, 1999).
Evidence of yield decline was noted in the United States as early as 1959. After ten years of research on many aspects of yield decline, researchers in the United States conceded, in 1969, that they were a long way from solving or completely understanding this complex problem (Schumann, Meyer & van Antwerpen, 2000). Research still continues on this subject.
In South Africa, poor sugarcane growth and low yields occur on about 60 percent of the area under sugarcane (Meyer, van Antwerpen & Meyer, 1996). Factors influencing sugarcane yield decline are often unclear and difficult to unravel (Ramburan, Wetttergreen, Berry & Shongwe, 2012).
2.10.2 Definition
Sugarcane monoculture farming is generally accepted as a viable practice. However the shortcomings of this farming system are becoming more apparent (Meyer & van Antwerpen, 2001). Sugarcane yield decline is widely defined as the reduction in productive capacity of soils under long-term sugarcane monoculture farming systems (Garside, 1997). This definition focuses on soil quality issues.
Jones et al (2012) offer a broader definition. They state that sugarcane yield decline is the decrease in productivity of land excluding the effects of changes in harvest age and climate. Climatic variability and harvest age effects can be excluded by using the Canesim41 crop model (Bezuidenhout & Singels, 2007).
41
37
2.10.3 StudiesIn South Africa, pre 2003, studies on the relationship between soil degradation and sugarcane yield decline were studied relatively one-dimensionally (South African Sugarcane Research Institute, 2004). Topics studied included (South African Sugarcane Research Institute, 2004):
Acidification (Schroeder, Robinson, Turner & Wallace, 1994)
Soil compaction (Swinford & Boevey, 1984)
Soil erosion (Platford, 1979 and 1982)
Water intake rate decline (Meyer, Dewey & Wood, 1988)
Irrigation water quality (Culverwell & Swinford, 1985)
Soil salinization (Johnson, 1978)
Soil surface crusting (Dewey & Meyer, 1989)
Soil organic matter (van Antwerpen et al, 1996).
The Australian sugar industry identified that sugarcane yield decline is a complex issue that was associated with many factors being out of balance in sugarcane farming systems. They adopted a multi- disciplinary scientific approach towards studying this problem by forming the Sugar Yield Decline Joint Venture in 1993. The Soil Sustainability Interest Group was formed in South Africa in 1998 by a small group of farmers, researchers, extensionists and trade representatives. It also adopted a multi- disciplinary approach to studying sugarcane yield decline (South African Sugarcane Research Institute, 2004).
Jones et al (2012) conducted a study using the Canesim crop model to quantify the rate of change in the productivity of sugarcane land attributable to factors other than climate variability and harvest age in the KwaZulu-Natal South Coast region. During the period 1980 to 2009, average actual yields per hectare decreased by 5.5 percent. Simulated yields over the same period, excluding the effects of reduced harvest age and climate showed a statistically significant higher yield decline of close to 11 percent per hectare. The effects of improved technology and better varieties were not considered in this study. These factors should have increased yields. The study concluded that yield decline contributed to reduced sugarcane production on the South Coast.
Yield decline is evident despite the improvement in yield potential from new varieties (Meyer, van Antwerpen & Meyer, 1996). Plant breeders have succeeded in partially compensating for yield decline. In Australia yield decline is occurring despite data suggesting that plant breeding programmes are increasing productivity by about 1 percent per annum (Australian Centre for International Agricultural Research, 2015).
38
Ramburan et al (2012) conducted four sugarcane trials in South Africa and Swaziland to examine the contributions of variety, growing environment (climate and soil) and management to sugarcane yield decline. They determined that the growing environment had an overriding effect on yield decline. This was followed by management practices. The effect of variety was of secondary importance. The implication of this study was that sugarcane farmers should place more emphasis on altering the growing environment through better management, than focusing their efforts on choosing a perceived superior variety.Singels, Donaldson and Smit (2005) conducted a study on BT1 at SASRI to determine if climate change was impacting on sugarcane yields. Actual yield data was compared to simulated data over the time period 1954 to 2004. The burnt, with tops spread and fertilised treatment plots were used. The authors found an 8 percent increase in yield over the past 50 years due to climate change. Temperatures increased by 0.75 degrees Celsius over the period.
If it was not for improved varieties and warmer temperatures, sugarcane yields would have been even lower than current levels.
2.10.4 Causes
As articulated previously, sugarcane yield decline occurs because of a reduction in the productive capacity of the soil due to a decline in soil quality (Meyer & van Antwerpen, 2001). Plant root development is affected which influences the general well-being of the sugarcane plant.
Most research on this topic has focused on the effects of long-term sugarcane production on soil quality (Haynes & Hamilton, 1999). As stated in the previous section the following aspects linking sugarcane yield decline to soil degradation have mostly been studied in isolation in South Africa - soil acidification, soil compaction, soil erosion, soil water infiltration rate, irrigation water quality, salinization, surface crusting and soil organic matter (South African Sugarcane Research Institute , 2004). Other factors that have played a role in sugarcane yield decline include excessive tillage, burning at harvest, soil structure breakdown and insufficient plant available nutrients (Meyer, van Antwerpen & Meyer, 1996; Haynes & Hamilton, 1999).
Varietal differences and stool damage may play a role in yield decline (Kingston, Donzelli, Meyer, Richard, Seeruttun, Torres & van Antwerpen, 2005). Long-term monocropped soils lose productivity. Inappropriate use of herbicides may also play a role in sugarcane yield decline (Sumner, 2011).
Sumner (2011) argues that for yield decline to occur a combination of, or a few factors are all that is necessary to precipitate decline. The combination can be different at different locations but the end result will be the same - a loss of yield. Site specific solutions are required (Jones, et al, 2012).
In Australia it was found that whilst root pathogens played a significant role in yield decline, other soil chemical, physical and biological factors played a role as well (Garside, 1997). Garside, Magarey and Braunack (2000) determined that the factors that caused yield decline varied in response to soil and the environment. Garside, Magarey and Braunack (2000) stated that the magnitude of yield decline
39
syndrome depends on the farming system, potential of the area and management. In high potential areas soil quality issues may be masked with high inputs. They suggested that more attention needed to be placed on improving soil quality in marginal growing areas in particular (Garside, Magarey & Braunack, 2000).2.10.5 Conclusion
Sustainable sugarcane production systems are needed to ensure the long-term viability of the sugar industry. These must maintain or improve soil quality and sugar yields (Meyer & van Antwerpen, 2001; Haynes & Hamilton, 1999).
Relying on plant breeding programmes to continuously breed more hardy plants to overcome poor management and deteriorating growing conditions is not a long-term sustainable agriculture solution. An alternative practical, cost effective and long-term solution may be to deal with the root cause of the problem by identifying the most limiting growth factors and correcting them.
Garside, Magarey and Braunack (2000) articulated that the magnitude of yield decline syndrome is heavily influenced by management and the farming system that is implemented. Ramburan et al (2012) determined that the focus of farmers should be on implementing practices that will alter the growing environment, rather than focus on quick fix solutions such as choosing varieties that are perceived to be better. If farming practices improve, soil quality improves and sugarcane varieties improve – then the farmer should be well along the path towards a sustainable long-term solution.
Maximum sustainable crop yields should be strived for. This is defined as: “The maximum crop yield
that can be sustained for a specific soil type and climate without undue harm to the environment.”
(Gregorich, Turchenek, Carter & Angers, 2001: 221). This can be interpreted to be the highest yield that can be taken from soil, under prevailing ecological and environmental conditions, without reducing the base of soil natural capital and compromising future yields. This crop yield must also not harm the long-term economic status of the farm or the needs of present and future generations who rely on the farm for sustainable livelihoods.