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Problem statement and impact of research
Naked crabgrass and large crabgrass are morphologically very closely related to each other. Maize producers often identify crabgrasses as large crabgrass irrespective of the knowledge that subtle differences exists between large crabgrass and the few unknown other Digitaria species, such as naked crabgrass. This leads to incorrect identification of grass weeds, subsequently leading to the assumption that all herbicides registered for control of large crabgrass will control these rather unknown Digitaria species as effectively. Although the weed status of large crabgrass has been studied worldwide, a lack of knowledge on naked crabgrass is quite evident as shown by the above literature review. Germination characteristics are species specific and as in the case with most grass species, seed of naked crabgrass showed dormancy and poor germination percentages with
initial trials. Different pre-treatments were tested to enhance the germination percentage of naked crabgrass and to shed light on conditions conducive to germination. The aggressiveness and competitive ability of naked crabgrass and large crabgrass was quantified, using a simple replacement series design, tested under two watering regimens. Maize is one of the largest cash crops produced in South Africa and the competition effect of both naked and large crabgrass was determined separately in greenhouse trials to quantify yield losses on two soil types. Since both grass weed and maize crop are C4 plants, severe competition for the same resources and similar growth patterns can lead to severe maize yield losses. Field trials were conducted over two growing seasons at two localities to determine the critical period of weed control of naked crabgrass to reduce maize yield losses to the minimum (Figure 1.6). Finally, herbicides registered to control large crabgrass effectively in maize were evaluated for control of naked crabgrass. Field trials and glasshouse trials were used to quantify control of both naked crabgrass and large crabgrass on two soil types. Research from this study focussed therefore, primarily on the biology, competition and control of naked crabgrass to clarify challenges producers experienced in some of the major maize producing areas in South Africa to increase sustainable control of this relative unknown Digitaria specie that can most probably become more dominant in the coming years. The hypothesis of this
study is therefore that the biology between related Digitaria species is not similar in all accounts and that the control measures for these grass weeds are species specific.
Competitive indices
If the AI=0, both species are equally competitive, while a positive value will signify dominance and a negative value will indicate the species that was dominated. A positive AI value was recorded for naked crabgrass root mass (0.3) and dominated large crabgrass (-0.3) significantly in the wet soil profile. In the dry soil profile the AI for root mass was however closer to zero (-0.07 and 0.07) for naked crabgrass and large crabgrass, respectively. The AI values for shoot and total biomass was not significantly influenced by the soil profile. The aggressivity of both species in the treatment mixtures is shown in Table 3.3 (AI values combined between soil profiles) for root, shoot and total biomass. Naked crabgrass dominated large crabgrass in the 3N:1S ratio for root, shoot and total biomass. Large crabgrass dominated naked crabgrass in the 1N:3S ratio only for shoot and total biomass. The AI in the 2N:2S ratio was equal to zero for all parameters indicating equal competitiveness between naked crabgrass and large crabgrass with regard to root, shoot and total biomass.
CHAPTER 1: LITERATURE REVIEW
1.1. History and description of Digitaria spp
1.2. Weed status and crop-weed competition
1.3. Germination characteristics
1.4. Control of crabgrass
1.5. Problem statement and impact of research
CHAPTER 2: Germination characteristics of the grass weed Digitaria nuda (Schumach.)
2.1. Introduction
2.2. Materials and methods
2.3. Results
CHAPTER 3: A comparison between relative competitive abilities of naked crabgrass (Digitaria nuda Schumach.) and large crabgrass (Digitaria sanguinalis (L.) Scop.) in two soil water regimens
3.1. Introduction
3.2. Materials and methods
3.3. Results
3.4. Discussion
CHAPTER 4: Comparative interference and competition status of naked crabgrass (Digitaria nuda Schumach.) and large crabgrass (Digitaria sanguinalis (L.) Scop.) on maize
4.1. Introduction
4.2. Materials and methods
CHAPTER 5 Critical periods of weed control for naked crabgrass (Digitaria nuda Schumach.), a grass weed in maize in South Africa
5.1. Introduction
5.2. Materials and methods
5.3. Results and Discussion
CHAPTER 6 Comparative efficacy of herbicides registered on maize to control naked crabgrass (Digitaria nuda Schumach.) and large crabgrass (Digitaria sanguinalis (L) Scop.)
6.1. Introduction
6.2. Materials and methods
6.3 Results
6.4. Discussion
CHAPTER 7 GENERAL DISCUSSION
