Physiological basis of herbicide interaction and integrated management of Palmer amaranth (Amaranthus palmeri)

Doctor of PhilosophyDepartment of AgronomyRandall S. CurrieMithila JugulamPalmer amaranth is a major threat to many cropping systems in the USA. As a result of selection, Palmer amaranth has evolved resistance to at least six herbicide modes of action including microtubule-, 5-enolpyruvylshikimate-3-phosphate synthase-, acetolactate synthase-, photosystem II-, hydroxyphenylpyruvate dioxygenase-, and protoporphyrinogen oxidase- inhibitors. Dicamba is effective for Palmer amaranth control; however, extensive use of this herbicide increases the likelihood of evolution of resistance to dicamba. The overall objective of this dissertation was to investigate the physiological basis of interaction of herbicides with different modes of action in Palmer amaranth control and evaluate use of integrated approaches to manage Palmer amaranth in field conditions. The specific objectives were to: 1) evaluate the effect of plant height on dicamba efficacy to control Palmer amaranth; 2) investigate the mechanism of resistance to glyphosate in a Palmer amaranth accession from Kansas, and evaluate efficacy of glyphosate and dicamba tank-mix to control this accession; 3) investigate the physiological basis of glyphosate and dicamba interaction in tank-mix to control Palmer amaranth; 4) determine the efficacy of reduced dicamba use on Palmer amaranth control in irrigated corn production; and 5) investigate grain sorghum and Palmer amaranth growth and reproductive attributes in response to sorghum density and nitrogen rate under irrigated conditions. All experiments were repeated and appropriate statistical tests were used for data analyses. The results indicate: a) increased absorption and translocation of dicamba contribute to increased efficacy to control Palmer amaranth at early growth stage; b) tank mixing glyphosate and dicamba had a synergistic effect on Palmer amaranth control; c) rapid absorption of dicamba and increased translocation of glyphosate resulted in increased Palmer amaranth control when applied in combination; d) there is an opportunity to maintain grain yield while effectively controlling Palmer amaranth in irrigated corn with the integration of increased corn plant population density and reduced dicamba application and e) integrating sorghum plant population and nitrogen did not suppress Palmer amaranth in irrigated sorghum, although sorghum grain yield was maintained. The outcome of this dissertation provides several strategies to improve control of Palmer amaranth

Cassava (Manihot esculenta Crantz) Tuber Quality as Measured by Starch and Cyanide (HCN) Affected by Nitrogen, Phosphorus, and Potassium Fertilizer Rates

Cassava (Manihot esculenta Crantz) is an important subsistence crop for many poor rural families in Africa. Cassava contains cyanogenic glucosides (linamarin and lotaustralin) which liberate hydrogen cyanide (HCN) during tuber processing. Once liberated, HCN attaches to the processed tuber. Continuous consumption of processed tuber containing high HCN concentration coupled with low protein intake causes Konzo – a paralyzing disorder that impacts children and women of childbearing age. There are ways to reduce HCN concentration during tuber processing; however, this can also reduce the overall starch content in the cassava tuber. A study comprising twenty treatments consisting of different combinations of nitrogen (N), phosphorus (P), and potassium (K) fertilizer rates was initiated in 2013 in the coastal Dondo District of Mozambique to assess cassava tuber quality as measured by starch and HCN. Significant differences were observed in starch content (CSC) of unprocessed tubers due to combined addition of N, P and K fertilizer rates, sample size, and estimation procedure. However, no significant differences were observed in HCN concentration in tubers due to the addition of N, P and K fertilizer. The HCN concentration in cassava tuber appears to be a function of the physiology of the crop or possibly cassava variety rather than the environment or conditions under which the crop is grown

INVESTIGATING THE EFFECT(S) OF CONTRASTING NITROGEN (N), PHOSPHORUS (P), AND POTASSIUM (K) FERTILIZER RATES ON CASSAVA TUBER YIELD AND QUALITY AND MAIZE GRAIN YIELD IN SOUTHERN AFRICA

Fertilizer is a major limiting factor to agriculture in southern Africa (SA). Coupled with this is lack of appropriate fertilizer recommendation rates for high productivity in existing agricultural systems. Field experiments were conducted to determine nitrogen (N), phosphorus (P), and potassium (K) fertilizer rates for high cassava tuber yield and quality for the coastal semiarid Dondo District of Mozambique, and high maize grain yields for both vertisol and inceptisol of Maphutseng in Lesotho. In general, the results showed that cassava tuber yield, cassava tuber quality as measured by tuber starch content, and maize grain yield were significantly increased by fertilizer addition (p0.05). The results also showed that maize grain yield was not significantly affected by tillage practices (p\u3e0.05). Combined applications of 60 kg N-60 kg P-0 kg K and 60 kg N-90 kg P-150 kg K kg per ha are suggested for high cassava tuber yield, and high cassava tuber starch content for the coastal semiarid Dondo District of Mozambique, respectively. Economically optimum maize grain yields (EOY) and profits (EOP) for the southwest lowlands of Maphutseng village, Mohale’s Hoek District, Lesotho, were estimated at 222 kg, 182 kg, and 123 kg of N (applied as limestone ammonium nitrate) per ha for no-till vertisol, no-till inceptisol, and till vertisol maize systems, respectively. The results suggest that an application of 30 kg of P (P2O5) per ha is required for high grain yields in inceptisol maize system. The results also confirmed that the benefits of not tilling the soil are not immediate. On the whole, the results suggest that there is potential to increase productivity in existing agricultural systems with the use of fertilizer in both Mozambique and Lesotho. However, this will not be possible without increasing farmers’ access to fertilizer given that fertilizer use in both countries is still very low (\u3c 20 kg per ha)