Current breeding methodology in white clover

The fundamental concept of plant improvement is selecting appropriate genotypes that possess characteristics that determine adaption and agronomic performance in the target environment. Unlike crop species where the major breeding objective is to increase grain yield, the objective in white clover breeding is not to solely maximise yield in a monoculture environment but instead to produce a balanced sward with a companion grass species and maintain a reliable, consistent white clover contribution which improves economic returns from animal products (Abberton and Marshall, 2005; Jahufer et al., 2002).

Progress in early white clover breeding programmes was achieved through improving performance within existing ecotypes. Cultivars such as Grasslands Huia proved to be commercially successful for several decades. Superseding cultivars were the result of hybridising persistent ecotypes with more agronomic imported material. The extent of variation within a particular breeding pool has not been large enough to allow rapid gains under selection for desired improvements, and the use of ex situ genetic resources has been critical in the development of new varieties (Abberton and Thomas, 2011). The development of adapted breeding pools paved the way for phenotypic recurrent selection programmes which by in large has become the most widely adopted breeding practice (Woodfield and Caradus, 1994).

The breeding success of a white clover cultivar is dependent on the selection criteria used in the programme, while simultaneously utilising realistic evaluation systems which simulate its

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intended farming environment (Evans et al., 1996). Mixed species pasture swards make sole improvement of the individual species challenging. Caradus et al. (1989) evaluated the relative merits of both mono-sward spaced-planted nurseries and small mixed species plot trials. Highly heritable traits such as leaf size showed significant correlation across a range of culture conditions due to the limited environment interaction of this trait. However, traits with low heritability such as agronomic performance showed poor correlation between spaced plant and mixed species plot trials (Caradus et al., 1989; Davies, 1970; Davies and Tyler, 1961).

Spaced planted trials have been considered to be relatively unreliable for estimates of

agronomic performance (Atwood and Garber, 1942; Davies and Tyler, 1961; Gibson, 1964). However, assessment of elite individuals is the basis of plant improvement. Caradus et al.

(1989) suggested that although performance of mixed species plots cannot be adequately predicted from spaced planted trials, there is reasonable correlation between top performing lines in spaced plant nurseries and top performing lines in mixed species plots to justify the use of a such a system in certain circumstances. However, undoubtedly the progressive movement in the mid to late 20th _{century to evaluating material in competitive swards has}

enhanced the ability of breeders to identify superior genotypes and populations (Woodfield and Caradus, 1994).

Variation in trial defoliation management between breeding programmes is well documented. Conclusive evidence demonstrates cultivar performance under cutting does not accurately reflect cultivar performance under intensive grazing (Dijkstra and Vos, 1972; Evans et al., 1992). Woodfield and Caradus (1994) suggested breeding programmes which continue to use mechanical defoliation may in fact degrade the agronomic performance of breeding material. The progress made in white clover over the past century has been largely attributed to the use of progressively more realistic screening and evaluation methods and to the maintenance of a wide genetic base (Woodfield and Caradus, 1994).

2.5.6 Breeding progress in white clover

Conventional plant breeding has steadily delivered improved white clover performance in New Zealand pastoral systems (Woodfield and Easton, 2004). The improvement of white clover performance lies between 6% to 15 % per decade, although the extent of improvement varies among clover leaf types (Woodfield, 1999; Woodfield and Caradus, 1994).

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In comparison, the reported genetic gains in other forage crops including perennial ryegrass

(Lolium perenne), Italian ryegrass (Lolium multiflorum Lam) and alfalfa (Medicago sativa L.)

are similar or inferior (Hill et al., 1988; Holland and Bingham, 1994; Vanwijk and Reheul, 1991). In alfalfa, tetrasomic inheritance limits the speed of genetic gain, given the increased number of possible allelic combinations at a single heterozygous locus (Hill et al., 1988). The gains achieved in white clover are the result of accumulating genes with improved yield potential, as well as reducing the effects of yield limiting factors (Woodfield et al., 2001). While genetic gains are superior for white clover within forage species, they are markedly lower than in many cereal species. Casler & Brummer (2008) summarised several reasons for the yield lag in forage crops relative to gain crops which included (i) a longer breeding cycle for forage crops, most of which are perennials, (ii) lack of a “harvest index” trait to aid dry- matter partitioning into the economic product, (iii) inability to exploit heterosis in commercial cultivars, and (iv) the focus on a wide array of economically important traits of forage crops, many which are not specially correlated or may even be negatively correlated with forage yield. The latter is well documented where selection response rapidly decreases as the number of uncorrelated traits increases (Fehr, 1987).

A considerable drawback with all forage species is the manipulation of harvest index. Whereas the harvest index in crop species can be subtly manipulated by partitioning more photosynthates to the grain at the expense of vegetative production, the harvest index in forage species is the total above ground biomass and only moderate changes in harvest index are possible through the redistribution of resources from the roots to shoots (Woodfield and Caradus, 1990).

While short term forage yield is important, it is not always considered first priority in white clover. White clover and perennial ryegrass swards may be used profitably for a number of years and reliability over time of the white clover contribution is more of a key concern of the farmer and, therefore, the breeder (Abberton and Marshall, 2010). Importantly, clover

contribution in grazed swards is directly related to animal live weight gain (Chapman et al., 1993). Increased animal performance has been the main objective in most white clover breeding programmes and therefore its associated parameters have become important breeding selection criteria (Woodfield and Caradus, 1994). These attributes include clover content in the sward, total sward yield, persistence and forage quality.

White clover persistence is a key focus of most international breeding programmes. The development of a strong network of stolons is a pre-requisite of persistence and therefore

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stolon characters have been a major focus of breeding efforts (Abberton and Marshall, 2010). The breaking of the negative correlation between leaf size and stolon density has long been a goal of breeders in an effort to increase persistence in larger more productive leaf cultivars (Woodfield and Caradus, 1994). Whilst winter hardiness has been of particular focus in UK and Scandinavia to increase persistence (Helgadottir et al., 2008), drought tolerance has long been a major objective in temperate environments with summer moisture stress. Progress has been made in both, but the latter proves somewhat more challenging.