Valuing forages for genetic selection: what traits should we focus on?

Abstract

Failure over the past two to three decades to implement industry-led, systematic forage evaluation systems that translate forage performance data to animal production and economics means that the livestock industries are poorly positioned to judge how much economic benefit they are gaining from forage plant improvement and to propose future priorities and targets. The present paper identifies several knowledge gaps that must be filled to enable the value being delivered to pasture-based livestock industries by forage improvement to be determined, demonstrated to farmers and increased in the future. Seasonal yield, total annual yield, nutritive value and feeding value of pasture are all important traits for driving the productivity of pasture-based livestock production systems. From a farm systems perspective, persistence of the yield or quality advantage of new cultivars is also economically important. However, this is the least well defined of the productivity traits considered in the paper. Contrary to anecdotal reports, evidence indicates that the genetic potential of modern ryegrass cultivars to survive in grazed pastures is at least equivalent to that of older cultivars. Plant breeding in Europe and New Zealand has changed the seasonal yield, quality and intake potential of perennial ryegrass. On the basis of dry matter (DM) yield data from small-plot evaluation trials, the New Zealand forage value index indicates that the top-ranked perennial ryegrass cultivars offer between NZ$280 and NZ $650/ha per year potential additional operating profit to dairy businesses (depending on region), compared with a historical genetic base of cultivars that were first entered into yield testing programs between 1991 and 1996. The equivalent figure in Ireland (including nutritive value effects) is about €325/ha per year. These estimates are yet to be confirmed in animal production studies. In intensive dairy systems, current rates of genetic gain in DM yield lag well behind realised rates of gain in animal genetics and associated increases in feed demand per animal. Genetic gains in yield need to double from current rates (estimated at 0.5% per year); but, it is not known whether this is possible in an outcrossing species such as perennial ryegrass, which is normally grown in a mixture with other species, especially white clover. Improvements in DM yield in seasons where extra DM has greatest economic value in grazing systems should dominate breeding objectives, but this must now be augmented by consideration of the environmental impacts of intensive pasture-based livestock production systems and opportunities to mitigate this through germplasm selection. There is less evidence that nutritive or feeding value of ryegrass cultivars significantly limits animal production and profitability and useful improvements have already been made using tetraploids and later heading material.

Field Spectroscopy to Determine Nutritive Value Parameters of Individual Ryegrass Plants

“…NV refers to multiple traits which contribute to the amount of energy and nutrients that can be obtained by grazing stock, thereby contributing towards the total liveweight gain or milk production of the animals [3]. There is some disagreement on the relative importance of various traits but most forage scientists agree that important traits include cell wall constituents such as acid detergent fibre (ADF), neutral detergent fibre (NDF), as well as the dry matter percentage (DM), crude protein (CP), in vivo dry matter digestibility (IVVDMD), in vivo organic matter digestibility (IVVOMD) and water soluble carbohydrates (WSC) [4][5][6][7][8][9][10]. Improvement of these NV traits would increase the amount of nutrition available for stock without increasing the yield and would decrease the need for costly supplements.…”

Using Ecophysiology to Improve Farm Efficiency: Application in Temperate Dairy Grazing Systems

“…Some of these traits, especially tetraploidy, are well-documented as facilitating greater dry matter intake [68] allowing better control of post-grazing sward state. Milk production benefits are likely from the use of such material [69], but this has not yet been confirmed in rigorous grazing systems experiments [70].…”

“…More targeted research is needed to quantify exactly how different modern cultivars are compared with older cultivars in terms of grassland productivity [70], and therefore some caution is required in when questioning whether current guidelines still result in best possible pasture harvest rates. Nonetheless, the emergence of greater genetic diversity through the development of an "extended phenome" of perennial ryegrass (combining genetic variation in the ryegrass plant and the endophytic fungus) may open opportunities for farmers to grow and utilise more pasture, thereby controlling the costs of milk production by reducing/avoiding the need to purchase expensive supplements.…”

Genomic Predictive Ability for Foliar Nutritive Traits in Perennial Ryegrass

“…The reduced emphasis on breeding for nutritive traits in forages is affected by a number of factors, including a lack of consensus on specific breeding targets (Wheeler and Corbett 1989;Chapman et al 2015), ambiguous evidence for the impact of specific nutritive traits on animal production outcomes (Easton et al 2002;Edwards et al 2007;Mcevoy et al 2011), the confounding influence of environment and genotype x environment (G x E) interactions, and the significant additional resources needed in a breeding program to undertake nutritive trait measurements in large panels of selection candidates (Smith et al 1997).…”