White clover (Trifolium repens L.) and red clover (T. pratense L.) are the most important legumes of temperate pastures. The former is used largely in systems based around sheep or cattle grazing and is grown together with a companion grass. Breeding aims to optimize the white clover contribution to the sward. This means that yield per se is not the aim but rather to take full advantage of the benefits of white clover; in particular, nitrogen fixation, high protein content, digestibility, mineral content and high intake. The objective is an agronomically and, as far as possible, nutritionally balanced sward, thus persistence of white clover and yield stability over a number of years are key goals. A considerable focus of germplasm improvement has therefore been overcoming biotic and abiotic stresses to clover performance. The former include not only pests and diseases but also the impact of the ruminant animal and the competitive interaction with the companion grass, while abiotic stress could be loosely defined as ‘winter hardiness’ and ‘summer survival’ depending on the site. In recent years the focus of many breeding efforts has shifted to give more consideration to the effects of variation within white clover germplasm on animal performance and the environment. Beneficial effects on productivity have been known for many years, but recent studies of the impact of forage diets on meat and milk quality have opened up new opportunities for improvement. Diffuse pollution of nitrogen and phosphorus from agricultural sources is high on the environmental protection agenda of many governments. Breeding efforts are now being made to reduce the contribution of clovers to both direct (leaching) and indirect (through animal returns) pollution. In particular, recent insights into mechanisms affecting protein breakdown in the rumen and silo offer new prospects for breeding interventions to reduce environmental impacts.
Molecular marker methods are being developed in white clover and the transfer and use of resources and information accumulating in the model legumes Medicago truncatula and Lotus japonicus is likely to be a major route by which the power of genomic approaches is translated into forage legume improvement. Hybrids of white clover and related species have been developed to introgress key traits; namely, drought tolerance, grazing tolerance of large leaf types and enhanced seed yield, for which only limited genetic variation is present within the white clover gene pool.
Red clover is less persistent than white clover, is typically cut three or more times in a season and is used to make silage for winter feed. Although it is often grown with a companion grass, monocultures are common and yield per se as well as persistency and pest and disease resistance are major breeding aims. Fewer agronomic studies and less germplasm improvement have been carried out in this species and molecular studies are not as well advanced although, as with white clover, future developments are likely to benefit greatly from a close relationship to model legumes. Red clover brings considerable benefits in terms of animal production and meat and milk quality. These aspects, alongside approaches to reduce nitrogenous pollution from the silo, represent considerable opportunities for variety development.