Evaluation of genetic gain and diversity in CIMMYT Southern Africa hybrids and open pollinated varieties tested in regional trials from 2000 to 2010

Abstract

English: Maize is a staple food crop in ESA, making it a priority in terms of production. Farmers face challenges in production when maize crops fail in the face of drought, limited or no access to fertiliser or lack of capital to purchase the fertiliser and when there are disease outbreaks. CIMMYT embarked on a drought breeding programme in 1975 that spread to Southern Africa in the 1990s. Since 1997 CIMMYT ESA has focused on producing drought tolerant varieties but incorporated into the selection screening for high yielders under optimal conditions and low N stress tolerance and MSV, GLS and ET disease resistance including. A study to evaluate 67 hybrid best yielders selected from an 11 year period covering 2000 to 2010 for genetic gain in grain yield, random and managed drought stress tolerance, low N tolerance and disease (MSV and ET) resistance was conducted from the 2011/12 main season to the 2013 off season. Parental lines of the 67 hybrids were evaluated for genetic diversity. Sixtyseven CIMMYT ESA hybrids released from 2000 to 2010, checked by three commercial hybrids, were evaluated for genetic gain in grain yield and secondary traits in a multi-site alphalattice designed experiment over two seasons. The hybrids were evaluated across 14 optimal sites in Zimbabwe, Zambia, Malawi, Kenya and Uganda; seven random drought sites in Chiredzi and Kadoma (Zimbabwe) and Potchefstroom in South Africa; four managed drought sites in Chiredzi, Zimbabwe and Kiboko, Kenya; five low N sites in Harare, Zimbabwe and Kiboko, Kenya and three MSV sites all in Harare, Zimbabwe. Significant gain in grain yield and stress tolerance was realised in the CIMMYT ESA maize breeding programme under optimal and stress conditions. Genetic gain in grain yield of 109.4 kg ha-1 year-1 (1.4% year-1), 20.9 kg ha-1 year-1 (0.62% year- 1), 141.3 kg ha-1 year-1 (2.2% year-1), 22.7 kg ha-1 year-1 (0.85% year-1) and 32.5 kg ha-1 year-1 (0.85% year-1) were realised under optimal, low N, MSV, random drought and managed drought stress respectively. This gain in grain yield was associated with phenological and physiological changes. The season length showed a tendency to increase across all environments at rates ranging from 0.048 d year-1 to 0.134 d year-1 but did not significantly change. The season length of the new hybrids was still within the length of the local checks, meaning that the new hybrids still fit in the normal season. The new hybrids had shorter ASI under all environments that insured better pollination and grain set. An increase in the number of ears per plant was recorded across all environments and this positively contributes towards increased grain yield and reduced barrenness. New hybrids were shorter in all environments except managed drought stress. A decrease in plant height indicate improved dry matter partitioning to the ear for ear development, silking, pollination, grain set and grain development and filling. Under random and -managed drought, low N and biotic stress newer hybrids were taller showing normal growth under stress while the old hybrids were stunted. Senescence was lower in new hybrids across all environments except managed drought. There was no change in the number of days to mid-anthesis. MSV and ET resistance or tolerance was higher in the new hybrids that had low disease scores and yielded better than the old hybrid. Overall the genetic gain in yield, drought stress tolerance, low N stress tolerance and biotic (MSV and ET) tolerance over the 11 years from 2000 to 2010 was high. The diversity of the parental lines was relatively low with genetic distances ranging from 0.006 to 0.4005 with most distances ranging from 0.2000 to 0.4000. Considering the 54 parental lines were from the CIMMYT Southern Africa breeding programme only, and that the lines are a subsample of the breeding programme being parents of a sample (only 67) of the produced CIMMYT hybrids, the diversity was narrow because the lines are from the same geographical origin selected in a defined system for adaptation to the local environment and needs. A few lines (four) out of the 54 were used more frequently in most of the hybrids. The same lines were parental lines for some of the other 50 lines. Despite the wide range of lines, using a few lines frequently in formation of most of the hybrids can potentially narrow down the diversity of hybrids in the region if the hybrids as the best performers, are disseminated to farmers. This will pose a risk in the event of outbreaks of new pests and disease. The CIMMYT ESA maize breeding programme for drought tolerance realised significant gains in yield and stress tolerance providing farmers with better yielding varieties that are more resilient to stresses.

Afrikaans: Mielies is ‘n stapelvoedsel in ESA, wat dit ‘n prioriteit maak in terme van produksie. Kleinboere staar baie uitdagings in die gesig as daar misoeste is as gevolg van droogte, beperkte of geen kunsmis of ‘n gebrek aan kapitaal om kunsmis te koop of wanneer daar siekte uitbrake is. CIMMYT het in 1975 begin met ‘n teelprogram vir droogtetoleransie, wat uitgebrei is na Suidelike Afrika in die 1990s. Sedert 1997 het CIMMYT ESA gefokus op die produksie van droogte tolerante variëteite, maar seleksie vir hoë opbrengs onder optimale toestande en lae N stremming toleransie, sowel as MSV, GLS and ET siekte weerstand is ingesluit. ‘n Studie van genetiese vooruitgang in die beste 67 basters van ‘n 11 jaar periode van 2000 tot 2010 in graanopbrengs onder toevallige en beheerde droogtetoestande, lae N toleransie en siekte (MSV en ET) weerstand is gedoen in die 2011/12 hoofseisoen tot die 2013 af seisoen. Die ouerlyne van die 67 basters is geëvalueer vir genetiese diversiteit. Sewe en sestig CIMMYT ESA basters vrygestel van 2000 tot 2010, met drie kommersiële kontroles, is geëvalueer vir genetiese vooruitgang in graanopbrengs en sekondêre eienskappe in ‘n multi-lokaliteit alfa-reghoek ontwerp oor twee seisoene. Die basters is oor 14 optimale lokaliteite in Zimbabwe, Zambië, Malawi, Kenia en Uganda; sewe toevallige droogte lokaliteite in Chiredzi en Kadoma (Zimbabwe) en Potchefstroom in Suid Afrika, vier beheerde droogtelokaliteite in Chiredzi, Zimbabwe en Kiboko, Kenia; vyf lae N lokaliteite in Harare, Zimbabwe en Kiboko, Kenia en drie MSV lokaliteite in Harare, Zimbabwe. Betekenisvolle toename in opbrengs en stremmingstoleransie is gerealiseer in die CIMMYT ESA mielie teelprogram onder optimale en stremming toestande. Genetiese verbetering in graanopbrengs van 109.4 kg ha-1 jaar-1 (1.4% jaar-1), 20.9 kg ha-1 jaar-1 (0.62% jaar-1), 141.3 kg ha-1 jaar-1 (2.2% year-1), 22.7 kg ha-1 jaar-1 (0.85% jaar-1) en 32.5 kg ha-1 jaar-1 (0.85% jaar-1) is gerealiseer onder optimale, lae N, MSV, toevallige en beheerde droogtetoestande onderskeidelik. Hierdie vooruitgang in opbrengs is geassosieër met fenologiese en fisiologiese veranderings. Die lengte van die groeiseisoen het toegeneem oor al die omgewings teen tempos’s wat van 0.006 d jaar-1 tot 0.134 d jaar-1 maar het nie betekenisvol verander het nie. Die verlengde seisoen was binne die groeiseisoen van die plaaslike kontroles, dus pas dit nog in die normale groeiseisoen in. ‘n Langer groeiseisoen verleng die graanvul periode wat opbrengs van die basters verbeter. Die nuwe basters het korter ASI onder alle omgewings gehad wat beter bestuiwing en graanset veroorsaak het. ‘n Toename in die getal koppe per plant is gesien in alle omgewings en dit dra positief by tot verhoogde graanopbrengs en verminderde onvrugbaarheid. Nuwe basters was korter in alle omgewings behalwe in beheerde droogtetoestande. ‘n Afname in planthoogte wys op verbeterde droëmateriaal toedeling na die koppe wat verbetering veroorsaak in kopontwikkeling, stuifmeeldraad ontwikkeling, bestuiwing, graanset en graanvul en graanontwikkeling. Die nuwe basters was langer onder beheerde droogte, met normale groei onder stremming, terwyl die ou basters verdwerg was. Blaarvrekte was minder in nuwe basters oor alle omgewings behalwe onder beheerde droogte. Onder beheerde droogte het die basters die verlengde seisoen gebruik, wat die graanvul periode verleng het, maar hulle het ‘n vinnige afdroog aan die einde van die seisoen gehad. MSV en ET weerstand en toleransie was hoër in nuwe basters, en hulle het laer siektetellings gehad, met hoër opbrengs as ou basters. Oor die algemeen was die genetiese vooruitgang in opbrengs, droogte toleransie, lae N stremmings toleransie en biotiese (MSV en ET) toleransie oor die 11 jaar van 2000 tot 2010 hoog. Die diversiteit in die ouerlyne was relatief laag en het gewissel van 0.006 tot 0.4005 met meeste afstande wat gewissel het van 0.2000 tot 0.4000. Omdat die 54 ouerlyne net van die CIMMYT Suidelike Afrika teelprogram kom, was die diversiteit min omdat die lyne uit dieselfde geografiese oorsprong geselekteer is en aangepas is vir die plaaslike omgewing en behoeftes. ‘n Paar lyne (vier) van die 54 is meer gereeld gebruik in meeste van die basters. Dieselfde lyne was ouerlyne vir sommige van die ander 50 basters. Ten spyte van die wye reeks lyne beskikbaar kan die herhaaldelike gebruik van ‘n paar lyne die diversiteit van die basters verminder in die gebied as hierdie basters as die beste produseerders, versprei word aan die boere. Dit veroorsaak ‘n risiko as daar nuwe siektes en insekte uitbreek. Die CIMMYT ESA mielie teelprogram vir droogte toleransie het betekenisvolle vooruitgang in opbrengs en stremmingstoleransie gemaak, en boere voorsien van basters wat beter opbrengs het met beter toleransie vir stremming.