Evaluating maize production potential of selected semi-arid ecotopes using a water balance model

Abstract

English: The quantitative evaluation of crop production potential is important for sustainable and wise land use as well as for food security where subsistence farmers are involved. It is of particular importance in arid and semi-arid areas where rainfall is marginal and variable. This study aims at making a quantitative evaluation of the maize production potential of the Glen/Hutton and Glen/Oakleaf ecotopes which are located at the Glen Agricultural Research Station in the semi-arid Free State Province of South Africa. The objective was to characterize the ecotopes, and to make long-term yield predictions with a yield prediction model using long-term climate data. A detailed profile description, soil analyses and an in situ drainage curve were made for the Glen/Oakleaf ecotope. Similar data for the Glen/Hutton ecotope was obtained from previous research work (Hensley et al., 1993; Hattingh, 1993; Hensley, personal communication, 2002). A neutron water meter (NWM) was calibrated for each horizon of the Oakleaf soil on the Glen/Oakleaf ecootpe. The plant available water (PAW), defined as the differences between the drained upper limit (DUL) and the lower limit (LL), for maize grown on the Glen/Hutton and Glen/Oakleaf ecotopes was 133 mm and 120 mm respectively. Considering a mature maize crop growing in summer on these two ecotopes, PAW can be defined as the difference between the crop modified upper limit (CMUL) and LL. Results for this parameter were 183 mm and 192 mm for the Glen/Hutton and Glen/Oakleaf ecotopes respectively. The reason for the relatively high value of the latter is its slower drainage rate, which enables the crop to extract more water while drainage proceeds between field saturation and DUL than in the rapidly draining Hutton soil. Yields measured on experiments on the two ecotopes for 12 seasons on the Glen/Hutton and 10 seasons on the Glen/Oakleaf ecotope indicate that these two ecotopes have similar production potentials. For the development of a yield prediction model it was necessary to find a way to estimate daily crop evapotranspiration (ET). Based on the semi-arid climate, soil morphological observations and results of soil analyses, deep drainage from these two maize ecotopes was considered to be negligible. Equations for predicting runoff from rainfall (P) were developed based on long-term runoff measurements made at nearby sites (Du Plessis and Mostert, 1965; Hensley, personal communication, 2002). Because of fairly good r² values (0.84 and 0.82) the equations can be considered as reliable enough for the purpose of this study. A procedure for estimating soil water content at planting, from the rainfall pattern during preceding fallow period and grain yield in the preceding season, was also developed based on measurements from previous research work (De Jager and Hensley, 1988; Hattingh, 1993). Using all this information it was possible to make a fairly reliable estimation of daily ET. Climate data was used to calculate daily potential evaporation (Eo) values. This enabled the degree of crop water stress to be defined as ET/Eo , on a daily basis. The maize growing season was divided into three stages i.e. the vegetative, flowering and seed filling stages. A stress index (SI), defined as the average ET/Eo value for each period, was then calculated. To develop an integrated stress index (ISI) for the growing season eight different methods of integrating the three SI values were formulated. Measured maize yields from experimental plots on the two ecotopes were available for 22 seasons (De Wet and Engelbrecht, 1962; De Bruyn, 1974; De Jager and Hensley, 1988; Hattingh, 1993). Integrated stress index values were then calculated for these seasons and correlated with the biomass yields. This made it possible to choose the best method of calculating the ISI value from the individual SI's. The ISI with the best correlation (r² = 0.69) was the one with formula ISI = (2A + 3B + 2C)/7, where A, B and C are the SI values of the three growth periods respectively. The equation to predict total biomass (Yb) is Yb = 15238 ISI + 1067 kg ha¹. The biomass prediction equation was used to generate maize yields for 80 seasons (1922/23 - 2001/02). Yb was converted to grain yield using a harvest index regression equation based on 38 yields from Glen for which both total biomass and grain yield had been measured. Four production techniques were compared, i.e., November planting with conventional tillage (CTN), January planting with conventional tillage (CTJ), November planting with in-field water harvesting and basin tillage (WHBN), and January planting with water harvesting and basin tillage (WHBJ). Cumulative probability functions (CPF's) of yields were computed for the four different production techniques. The CPF's indicated that the long-term mean yields (at 50% probability) were 2653, 2 685, 3 108, and 3 355 kg ha¹ for CTN, CTJ, WHBN and WHBJ respectively. The CPF's were compared using the stochastic dominance and the Kolmogorov-Smimov (K-S) tests (Anderson et al., 1977; Steel et al., 1997). Stochastic dominance results indicated that the WHBJ and WHBN production techniques have well defined first degree stochastic dominance over the CTN and CTJ techniques. January planting showed only second degree stochastic dominance over November planting. The K-S test indicated that the CPF's of the water harvesting techniques were significantly different from those of the conventional production techniques. No statistical significant difference was observed with the K-S test between the November and January plantings.

Afrikaans: Kwantitiewe evaluering van gewasproduksie potensiaal is belangrik vir volhoubare grondgebruik en voedselsekuriteit waar kleinboere betrokke is. Dit is veral belangrik in ariede en semi-ariede gebiede waar reënval marginaal en wisselvallig is. Die doel van hierdie studie was om so 'n evaluering te maak vir mielies op die Glen/Hutton en Glen/OakleaJ ekotope geleë op die Glen Landbounavorsingstasie in 'n semi-anede gebied in die Vrystaat Provinsie van Suid-Afrika. Die doel was om die ekotope te karakteriseer, en om lengtermyn oesopbrengs voorspellings te maak met behulp van 'n opbrengsmodel saam met langtermyn klimaatdata. 'n Gedetaileerde profielbeskrywing, grondontledings en veldbepaalde dreineringskurwe is gemaak vir die Glen/OakleaJ ekotoop. Vergelykbare inligting vir die Glen/Hutton ekotoop is verkry van vroeër navorsingswerk (Hensley et al,. 1993; Hattingh, 1993; Hensley, personal communication, 2002). 'n Neutron watermeter (NWM) is gekalibreer vir elke horison van die wortelsone van die Oakleaf grond. Die veldbepaalde plantbeskikbare water (pA JJ? vir mielies, gedefineer as die verskil tussen die gedreineerde boonste grens (DUL) en die onderste grense (IL), was 133 mm op die Glen/Hutton ekotoop en 120 mm op die Glen/OakleaJ ekotoop. Met 'n volwasse gewas op hierdie ekotope in die somer kan PAW gedefineer word as die verskil tussen 'n gewasaangepaste boonste grens (CMUL) en LL. Resultate vir hierdie parameter vir mielies op die Glen/Hutton ekotoop is 183 mm, en vir die Glen/OakleaJ ekotoop 192 mm. Die rede vir die relatiewe hoë waarde van laasgenoemde is die baie stadiger tempo van dreinering wat dan toelaat dat die mielies meer water bokant DUL kan ekstraheer terwyl dreinering nog plaasvind. Opbrengste gekry met veldproewe op die twee ekotope vir 12 seisoene op die GlenlHutton en 10 seisoene op die Glen/OakleaJ ekotoop dui daarop dat die produksiepotentiaal vir mielies op die twee ekotope min of meer dieselfde is. Vir die ontwikkeling van 'n opbrengsmodel was dit nodig om 'n prosedure te vind om daaglikse evapotranspirasie (ET) te beraam. Gebaseer op die morfologie en ontledings van die twee profiele, asook die semi-ariede klimaat, is daar besluit dat diep dreinering gewoonlik weglaatbaar klein sal wees. 'n Prosedure om afloop te voorspel vanaf reënvaldata is ontwikkel met behulp van langtermyn afloopbepalings gemaak deur Du Plessis en Mostert (1965) op 'n naasliggende terrein, asook ander plaaslike afloop bepalings (Hensley, persoonlike kommunikasie, 2002). Weens redelik goeie r² waardes van 0.84 en 0.82 kan die ontwikkelde vergelykings beskou word as betroubaar genoeg vir die doel van die studie. 'n Prosedure om grondwaterinhoud by plant is ook ontwikkel. Dit is gebaseer op die reënvalpatroon gedurende die vooraJgaande braakperiode, die graanopbrengs van die vorige groeiseisoen, en relevante resultate van vorige navorsing. Al hierdie inligting het dit moontlik gemaak om redelik betroubare voorspellings van daaglikse ET te maak. Klimaatdata is gebruik om daaglikse potensiële verdamping (Eo) te bepaal. Dit het dit moontlik gemaak om die mate van gewaswaterstremming, gedefinieer as ET/ Eo , op 'n daaglikse basis te beraam. Die mieliegroeiseisoen is in drie groeiperiodes gedeel, naamlik, vegatatiewe-, blom- en saadvulperiode. 'n Stremmingsindeks (SI), gedefinieer as die gemiddelde ET/ Eo waarde vir elke groeiperiode, is dan bereken. Agt verskillende formules is voorgestelom 'n geïntegreerde SI waarde (ISI) vir die groeiseisoen te bepaal. Gemete mielieopbrengste op die twee ekotope vir 'n totaal van 22 groeiseisoen is beskikbaar (De Wet & Engelbrecht 1962; De Bruyn, 1974; De Jager & Hensley, 1988; Hattingh 1993). Agt verskillende ISI waardes is bepaal vir elkeen van hierdie seisoene en gekorreleer met die biomassa opbrengs. Die ISI met die beste korrelasie (r² = 0.69) was die een met die formule ISI = (2A + 3B + 2C)/7, waar A B en C die SI waardes is vir die drie groeiperiodes. Die vergelyking om biomassa te voorspel van ISI is Yb = 15238ISI + 1067 kg totale biomassa per ha. Die genoemde vergelyking, saam met langtermyn klimaatdata om ISI waardes te bepaal, is gebruik om mielieopbrengste vir 80 seisoene (1922/23 - 2001/02) te simuleer. Yb is omgesit na graanmassa met behulp van 'n oesindeks regressievergelyking gebasseer op 38 oesresultate by Glen waar albei totale biomassa en graanmassa bepaal is. Vier produksietegnieke is vergelyk, naamlik, (a) plant in November met konvensionele bewerking (CTN); (b) plant in Januarie met konvensionele bewerking (CTJ); (c) plant in November met in-land afloopopgaring met bakkiesbewerking (WHBN); (d) plant in Januarie met in-land afloopopgaring met bakkiesbewerking (WHBJ). Kumulatiewe waarskynlikheidsfunksies (CPF's) van graanopbrengs is bereken vir elke produksietegniek. Die volgende resultate is verkry: langtermyn gemiddelde graanopbrengste vir die vier behandelings met 'n 50% waarskynlikheid was 2653, 2685, 3108 en 3355 kg ha¹ vir CIN, CTJ, WHBN en WHBJ respektiewelik. Die CPF's is statisties vergelyk deur middel van die stogastiese dominancy en Kolmogorov-Smimov (/(-S) toetse (Anderson et al., 1977: Steel et al., 1997). Eersgenoemde toets het aangedui dat WHBJ en WHBN goed gedefineerde eerste orde stogastiese dominansie het oor CTN en CTJ, met die Januarie-plant behandelings slegs met tweede orde stogastiese dominansie oor die November-plant behandelings. Die K-S toets het aangedui dat die twee WHB tegnieke statisties betekenisvol beter was as die twee CT tegnieke, maar dat daar geen betekenisvolle verskille was tussen die Januarie-plant en November-plant behandelings nie.