While tetraploid plants of red clover are taller, have thicker stems, and have broader leaves that altogether result in a higher forage yield compared to diploids, they generally have substantially lower seed yields than diploid plants. Tetraploid red clover can be induced chemically by colchicine or nitrous oxide (N2O) and sexually by union of unreduced gametes. The average seed yield of tetraploid red clover in Norway is 60% of the diploid yield, while in Sweden it is 75%. One objective of this paper was to examine whether there is a difference in seed yield among chromosome doubled tetraploids and crossed tetraploids. A second objective was to investigate differences in seed yield and seed yield components in Norwegian and Swedish tetraploid populations. The third objective was to study which yield component most correlates with the seed yield per hectare. Seed production experiments were established at Landvik and Bjørke in Norway and Svalöv and Lännäs in Sweden. Populations made by crossings of tetraploids gave significantly greater yield (p < 0.001) compared to populations that were made by chromosome doubling. On average, Norwegian and Swedish varieties had equal yields in both experimental years. Norwegian and Swedish varieties differed mostly in earliness traits. Swedish populations began flowering on average 4 d earlier than Norwegian populations. Genotypic correlations showed that seed yield per flower head was the component with the highest correlation (r = 0.956 and r = 0.977) with yield per hectare in both experimental fields. Results from the second experimental year indicate a trend towards improved seed yield after several cycles of recurrent selection for higher seed yield per flower head.

R ed clover (Trifolium praTense L.) is one of the most widely grown forage legumes in temperate areas, together with alfalfa (Medicago sativa L.) and white clover (Trifolium repens L.) (Boller et al., 2010). Red clover is cultivated in mixtures with grasses on approximately four million hectares worldwide (Isobe et al., 2014). Red clover in grass mixtures is important because its nitrogen-fixing ability, through symbiosis with Rhizobium leguminosarum biovar trifolii, reduces the need for nitrogen (N) input in fertilizers (Taylor and Quesenberry, 1996). Additionally, high content of crude protein and polyunsaturated fatty acids in red clover makes this species a popular component of legume-grass mixtures (Taylor and Quesenberry, 1996;Lee et al., 2009). Moreover, the high content of polyphenol oxidase (PPO) in red clover slows down the degradation of proteins in silage and thus reduces N losses from plant tissues (Lee et al., 2004;Sullivan and Hatfield, 2006).Forage legume cultivars often produce low seed yields or seeds of low viability. Both diploid and tetraploid red clover cultivars ABSTRACT Satisfactory seed yield of red clover (Trifolium pratense L.) cultivars is crucial for the availability of seeds on the market. Many breeders and researchers have used seed yield components to measure, compare, and explain differences in seed yield between diploid and tetraploid red clover cultivars and populations; however, the relative importance of each component varies between studies. In 2011 and 2012, single-plant trials with several tetraploid and one diploid red clover cultivar were established at the Norwegian plant breeding station at Bjørke. The goal was to study the impact of different seed-yield components on the seed yield of tetraploid plants. Seed weight per flower head was the seed-yield component that correlated best with the seed yield plant −1 (r = 0.91 and r = 0.68 in 2011 and 2012, respectively). Path coefficient analysis has also shown that the seed weight per flower head had the highest direct impact on seed yield plant −1 (direct path coefficients were 0. 867 and 0.783 in 2011 and 2012, respectively). In comparison, the direct path coefficients for calculated number of flower heads, which was previously highlighted as the most important seed-yield component, were lower and more variable (0.739 and 0.392 in 2011 and 2012, respectively). Since previously seed yield per flower head was also identified as the most important seed-yield component in dense plant canopy, this component might have the potential to select for improved seed yield of new cultivars based on single plants. However, further studies are required to confirm this conclusion.

Red clover is a valuable forage crop, but often copes with unsatisfactory seed yield. Management practices to increase seed yield include fertilization, adequate weed and pest control, the synchronization of flowering through pre-cutting in spring, and the application of plant growth regulators to prevent lodging. Seed yield problems may have variable underlying reasons, such as inadequate pollination, fertility or genetic problems, or a combination of such traits. In this review paper, we summarize the results of recent publications that shed new light on the traits explaining seed yield differences between red clover genotypes. The main seed yield components are the number of flower heads per plant and the seed yield per flower head. Seed yield differences between diploid and tetraploid red clover are largely explained by the lower seed number per head. Recent research showed that, although inadequate pollination can lead to poor seed yield in certain areas, pollination cannot explain differences in seed yield between genotypes, cultivars, or ploidy levels. Correspondingly, corolla tube dimensions are not associated with seed yield, in spite of what is often believed by seed producers. On the other hand, fertility problems such as aberrations during male meiosis tend to occur more frequently in tetraploid genotypes and/or genotypes with low seed yield. A recent genetic study revealed 34 candidate genes for seed development, which opens perspectives for marker-assisted breeding. A final and remarkable finding is the occurrence of self-fertility in tetraploid red clover and its association with high seed yield. Breeders should be aware that selection for seed yield in tetraploid red clover may lead to unintentional selection for self-fertility, with possible consequences for inbreeding. The implications of recent findings for seed yield breeding and for the creation of novel tetraploids are discussed. Future research opportunities are considered.