Past lessons and future prospects: plant breeding for yield and persistence in cool‐temperate pastures

Abstract

Pastoral-based animal production systems are under increasing pressure to provide the high quantity and quality of feed needed for optimal ruminant performance. The capacity of farmers to increase forage yield further, solely by increasing fertilizer inputs or through improved pasture management, is limited. Emerging requirements to balance industry production targets against the need to reduce greenhouse gas emissions and N losses pose further challenges. Plant breeding is being asked to deliver results more urgently than at any time previously, and this review attempts to highlight issues that might limit the prospects for future progress by seeking lessons from four past examples: (i) white clover breeding gains and the need to consider the complexity of the grazed grass-clover mixed sward, with its tendency for cycling in plant species composition; (ii) a systems field trial of new and old grass ⁄ clover cultivars, and how the complexity of growth of perennial forage crops, and the dynamic optimality required for sustainable harvesting might limit our ability to breed for 'yield' per se; (iii) the manipulation of a physiological trait (low 'maintenance' respiration) and the implications of such changes for plant fitness and G · E interactions; and (iv) an hypothesis-driven development of a trait (high-sugar grasses) and the value of 'proof of concept' studies, the requirement of scientific understanding of the mechanisms of trait expression, and how one might in future go about assessing breeding achievements. We discuss the general ecological considerations around shifts in the frequency distribution of traits in new populations, whether altered conventionally or by genetic modification, and how selection for a particular trait might inadvertently reduce both fitness and persistence. A major priority for breeding, we propose, might be to revisit previously abandoned traits that affected the physiological performance of forage species, armed now with a capacity to monitor gene expression at the molecular level, and so unravel ⁄ control the G · E interactions that limited their benefits. We also discuss how a 'loss of yield advantage' of new cultivars, seen when tested several years after sowing, requires urgent investigation and propose this might be associated with fitness costs of perenniality. Finally, we argue for a careful reconsideration of what are realistic expectations for systems field trials and that focus on forage breeding might be shifted more to 'proof of concept' studies, critical experimental design, comparing 'traits' rather than 'cultivars', and the wider ecological assessment of fitness and function of traits in the plant, community and ecosystem.

Genetic gain in perennial ryegrass (Lolium perenne) varieties 1973 to 2013

“…While overall DM yield gain is significant across varieties registered on RLs over the past 40 years, gains in DM production in pasture deficit periods such as spring and autumn may well be viewed of higher economic value by farmers for commercial use especially as pasture performance comes under increased pressure due to limitations in land availability and environmental constraints in the future (Parsons et al 2011), Sampoux et al (2011 identified over 40 years, gains in DM yield of perennial ryegrass varieties released on European National lists were primarily in the summer and autumn but no gain was achieved in spring DM yield production where pasture supplementation requirements are at their highest. Gain in DM yield may very well need to be assessed on such pasture growth deficit periods in the year to ensure farmers are reaping maximum economic benefit from newly bred recommended varieties.…”

“…Limitations to land availability coupled with increasing environmental requirements to reduce greenhouse gas emissions and N losses to ground water are placing increased pressure on grass based animal production systems to provide additional quantities of high quality forage. The ability of farmers to increase forage yield, through increased fertiliser inputs is limited (Parsons et al 2011). Thus, increased performance must be achieved by other means and one of the most important avenues is grass breeding.…”

Simple versus diverse pastures: opportunities and challenges in dairy systems

“…The constraints the pasture base places on the reliability of supply of a high nutritive value diet supportive of milk production has been long recognised (Jacobs and McKenzie 2003). Much research effort over the past decade has been directed at addressing this challenge through the integration of other forage species into a predominantly perennial ryegrass feedbase (Farina et al 2011, Tharmaraj et al 2014, or trying to improve the seasonality or total production of perennial ryegrass (Parsons et al 2011. A recent review of these activities identified that this strategy was most successful when the additional species/strategy was perennial focussed, directly grazeable and responsive to N fertilizer and irrigation inputs.…”

Breeding for genetic improvement of forage plants in relation to increasing animal production with reduced environmental footprint

“…The energy supplied by a feed is a function of its digestibility, which has traditionally been a major target for plant breeding programmes (Kingston-Smith and Thomas, 2003;Parsons et al, 2011a). Increasing the digestibility of feed DM is a good way of increasing its energy use efficiency, although other factors are increasingly important forage breeding targets (Casler and van Santen, 2010).…”

“…Rumen ammonia concentrations are more constant during the day with increased WSC concentrations in the grass (Lee et al, 2002). In fresh grass-based diets, there is a curvilinear relationship between the dietary ratio of WSC to nitrogen and the proportional excretion of dietary nitrogen in urine (Parsons et al, 2011a). This relationship suggests that a high diet WSC : N ratio will increase the efficiency of nitrogen use by dairy cows, even if milk production is not affected.…”