Breeding strategies for Brachiaria spp. to improve productivity – an ongoing project

Two strategies have been used in the breeding of Brachiaria (syn. Urochloa) to produce new cultivars. The first involves exploring the natural variability existing in nature, which is the selection of ecotypes, mostly apomicts, from the diversity in germplasm banks. This strategy proved efficient originally and the cultivars in use in Brazil were derived in this way, but progress with this strategy is limited in the medium to long term. The generation of novel variability through crossings is the alternative strategy to continue producing new improved varieties. This has been possible only through artificial chromosomal duplication of diploid and sexual B. ruziziensis and the fact that this species is cross-compatible with B. brizantha and B. decumbens, the 2 most important species for tropical pastures. Crossings between species have been carried out at Embrapa Beef Cattle, Campo Grande, MS, Brazil, since 1988 (Valle et al. 1993, 1999, 2001, Resende et al. 2002). The initial success in obtaining interspecific hybrids in Brachiaria is indisputable and also helped establishing a practical methodology as well as allowing for the identification of apomictic parents with good combining ability. However, a comprehensive genetic improvement strategy to benefit from gains in quantitative characteristics associated with the majority of agronomic traits of economic interest in these species had not been started. Therefore, 3 methodologies are being used for this purpose: reciprocal recurrent selection; intra-population recurrent selection; and directed crosses between sexual and apomictic plants. The last 2 strategies are already underway and the strategies for agronomic evaluations are reported here.


Introduction
Two strategies have been used in the breeding of Brachiaria (syn.Urochloa) to produce new cultivars.The first involves exploring the natural variability existing in nature, which is the selection of ecotypes, mostly apomicts, from the diversity in germplasm banks.This strategy proved efficient originally and the cultivars in use in Brazil were derived in this way, but progress with this strategy is limited in the medium to long term.
The generation of novel variability through crossings is the alternative strategy to continue producing new improved varieties.This has been possible only through artificial chromosomal duplication of diploid and sexual B. ruziziensis and the fact that this species is crosscompatible with B. brizantha and B. decumbens, in Brazil the 2 most important species for tropical pastures.
Crossings between species have been carried out at Embrapa Beef Cattle, Campo Grande, MS, Brazil, since 1988 (Valle et al. 1993(Valle et al. , 2001;;Valle 1997;Resende et al. 2007).The initial success in obtaining interspecific hybrids in Brachiaria is indisputable and helped establish a practical methodology as well as allowed for the identification of apomictic parents with good combining ability.However, a comprehensive genetic improvement strategy to benefit from gains in quantitative characteristics associated with the majority of agronomic traits of economic interest in these species had not been started.Therefore, 3 methodologies are being used for this purpose: reciprocal recurrent selection; intra-population recurrent selection; and directed crosses between sexual ___________ Correspondence: Cacilda Borges do Valle, Embrapa Gado de Corte, Avenida Rádio Maia, 830, Zona Rural, Campo Grande CEP 79106-550, MS, Brazil.Email: cacilda.valle@embrapa.brand apomictic plants.The last 2 strategies are already underway and the strategies for agronomic evaluations are reported here.

Intra-population recurrent selection
Twenty-one sexual progeny from agronomic evaluation of an experiment involving 1,000 individuals were recombined.Mode of reproduction was confirmed prior to planting to make sure all plants were sexual.The planting was done in December 2011, using previously prepared cuttings placed in an intercrossing area (Figure 1a) arranged in randomized blocks, with 8 replicates and square plots of 1 m x 1 m.This area was isolated physically and spatially to avoid external pollen contamination from apomictic plants.In addition, periodic maintenance of the crossing and surrounding areas was performed to eliminate possible compatible signal grasses, different from those that were being recombined.
Two harvests of seed were performed: second week of April, and first week of May 2012; and the half-sib progeny produced are now being evaluated in replicated plot experiments (Figure 1b).Dormancy of seed was broken and seeds were germinated in plastic tubes of 280 cm 3 in October 2012.For each progeny, 108 tubes were planted.After 75 days, 50 plantlets of each progeny were transplanted to the field in an experiment arranged in randomized blocks with 10 replicates; each plot contained 5 plants, spaced 1.5 m between rows and 1.5 m between plants within rows.In addition to the 21 progeny of half-sibs, 5 control varieties were used, resulting in 1,300 plants, which are being evaluated individually for leaf dry matter production, total dry matter production, leaf:stem ratio, regrowth capacity and nutritional value.

Directed crosses between apomicts and sexuals
Crossings between 5 apomictic parents -Brachiaria brizantha (cvv.Marandu and Paiaguás, and B4), B. decumbens (cv.Basilisk) and the interspecific hybrid cv.Mulato IIand 4 sexual parents (BS09, BS15, HBGC336-T1 and HBGC336-T2) were made.These parents are divergent for characters of economic interest, such as: high productivity, resistance to spittlebugs, drought tolerance, tolerance of toxic aluminum levels and high nutritive value.The crossings were carried out both in the field and greenhouse, which resulted in successful production of all possible combinations.As for half-sibs, seed dormancy of these full-sib progeny was broken before seeds were germinated in tubes of 280 cm 3 at the beginning of October 2012.Again, 108 tubes per progeny were prepared.Seventy days after planting, the best 50 plantlets of each progeny were selected to be evaluated agronomically.
The experiment was arranged in randomized blocks with 10 replicates, each plot containing 5 plants, spaced 1.5 m between rows and 1.5 m between plants within rows.In addition to the 20 progeny of full-sibs, the 9 parents were planted as controls resulting in 1,450 plants, which are under evaluation for leaf dry matter production, total dry matter production, leaf:stem ratio, regrowth capacity and nutritional value.

The way forward
As the experiments were established between January 10 and January 16, 2013, only 2 harvests have been completed at the time of writing.At least 7 harvests will be carried out (2 in the dry season and 5 in the rainy season) in order to select superior hybrids to continue breeding.It should be possible to estimate the number of half-sib progeny that are superior to the cultivars.In the case of full-sib progeny, in addition to the superiority in relation to parent material, estimates of heterosis and combining ability will be performed.In both groups of progeny, after selection for leaf dry matter production, total dry matter production, leaf:stem ratio, regrowth and nutritional value, the progeny will be subjected to selection for resistance to biotic (spittlebugs) and tolerance of abiotic (drought and flooding) stresses.
The sexual progeny selected by these strategies will be used to continue the intra-population recurrent selection program.Among the full-sibs, it is expected that half will be apomictic and the superior genotypes will be candidates for new cultivars.
Figure 1.(a) Intra-population crossing block; (b) spaced plant half-sib progeny to evaluate individual plant performance.