The article presents the results of a study in 2021 of 13 tetraploid selective varieties of alsike clover (Trifolium hybridum L.) from Norway, Germany, Sweden, Denmark, Canada, Latvia, Belarus and Russia in terms of the productivity of air-dry matter of the standing crop and seed yield. The observations were carried out in a greenhouse experiment set up in the greenhouse complex of the Federal Williams Research Center of Forage Production & Agroecology. The purpose of the research is to choose for crossing and further breeding work the most productive tetraploid clover genotypes from different geographical regions in terms of standing crop and seed yield. The criterion for selecting promising selective numbers for crossing was the deviation of the standing crop yield and (or) seed yield by more than 3 standard deviations (σ) from the corresponding arithmetic mean in the experiment. For experimental data on the yield of air-dry matter of the standing crop, a normal distribution was characteristic, and for seed yield, an exponential distribution. This made it possible to use the critical value of 3σ in making breeding decisions. The average productivity of air-dry matter of the standing crop is 47 g/vessel, σ=16 g/vessel. The average seed productivity is 1.1 g/vessel, and σ=0.9 g/vessel. According to the value of air-dry matter of the standing crop of 102 g/vessel, selective number 56 (cv. Tetraploid from the Republic of Belarus) was identified, and according to seed yield of 4.2 g/vessel, selective number 42 (cv. Alpo from Norway). They belonged to the productivity of Novator variety 213% and 233%, respectively. Selective numbers 56 and 42 are promising for crossing and studying the ability of offspring to combine high fodder and seed productivity.

The article presents the results of breeding work (2004–2018), aimed to create a highly winter hardiness hay-pasture variety of alsike clover (Trifolium hybridum L.) KB-4M, which forms 9–11 t/ha of dry matter in 2–3 mowings, with a seed productivity 130–150 kg/ha. The breeding population KB-4M was created in the Federal Williams Research Center of Forage Production and Agroecology. This breeding population is a complex hybrid population created as a result of free-confined pollination of tetraploid genotypes. The genotypes of 'Krasavik' variety are female parents. 'Krasavik' variety was created in the Research and Practical Center of National Academy of Sciences of the Republic of Belarus for Arable Farming. Pollinating varieties: 'Alpo' (Norway), 'Högsta' (Sweden), 'Eutetra' (Germany). The breeding population KB-4М is mid-ripening. Period from spring growth till first cutting is 37–47 days, till seeds ripening — 112–120 days. The winter hardiness is quite high (90.0–99.8%). During the period of competitive variety testing (2014–2016) the breeding population KB-4M has demonstrated an advantage over the standard 'Pervenets' in terms of the main economically valuable properties. For the three testing cycles have been demonstrated the following characteristics of yielding ability: green mass — 48.8–64.3 t/ha (9.9–24.6% more than the standard), hay — 9.14–12.37 t/ha (18.2–23.3% more than the standard), seeds — 130–150 kg/ha (18.2–22.7% more than the standard). The nutrient value: crude protein, depending on the cut, 169–223 g/kg dry matter, crude fiber — 139–275 g/kg. In 2020, the breeding population KB-4M was transferred to the state variety testing. This breeding population recommended for hayfields and pastures in all regions of alsike clover cultivation, especially for heavy, loamy, wet soils with high acidity (pH less than 5.5) and thin humus-layer, as well as for: grassing and land resting, cultivation of bog soils and peat bogs, establishment of melliferous crops.