Breeding objectives and evaluation methods

In 2011 A. Stewart and Hayes (2011, p. 32) stated:

The forage breeder’s goal is to develop cultivars that will improve animal performance on farms.

For this to happen, cultivars need to be productive, they have to be able to produce seed to be delivered to farmers and they also have to help minimise the impact of the production system on the environment (A. Stewart & Hayes, 2011). Traits such as total annual herbage yield, seasonal

distribution of herbage yield, herbage quality, persistency as well as resistance to pests and diseases, and tolerance to freezing, drought and heat, are the main targets in perennial ryegrass breeding (Lee et al., 2012; A. Stewart & Hayes, 2011; Wilkins, 1991; Woodfield & Easton, 2004). A. V. Stewart

(2006), summarising previous findings, indicated that genetic gains for annual yield were estimated at between 0.25 and 1.5 % per year, but also pointed out that an increase in total mixture yield could be limited by the partial suppression of clover due to the increased yield of the ryegrass component. To alter the seasonal distribution of yield, breeders have manipulated heading date of ryegrass (Lee et al., 2012; Wilkins, 1991). Earlier heading cultivars produce more feed in late winter while later cultivars provide higher quality herbage in late spring (Easton et al., 2002). Feed quality is an important factor in achieving improved animal performance.

High nutritive value means high DM digestibility and ME density, easy breakdown of forage into small particles by chewing, high non-structural carbohydrate content and high protein content (Lambert & Litherland, 2000; Wilkins, 1991). To improve herbage quality, breeders have also manipulated flowering behaviour, both timing of the main period in spring and the aftermath that occurs in summer. Retarding this process maintains quality of herbage longer during spring (Lee et al., 2012). Tetraploidy has also been used to improve quality (A. Stewart & Hayes, 2011). Doubling chromosome number by the use of colchicine (Morgan, 1976) has created cultivars with increased tiller, root and seed size, with larger cells, larger leaves, longer extended tiller height, and increased water soluble carbohydrate yield, but with lower tiller density and dry matter content (Lee et al., 2012;

Neuteboom, Lantinga, & Wind, 1988; Wilkins, 1991). Breeding has also successfully increased the water soluble carbohydrate content of cultivars, creating ‘high sugar grasses’ that help to mitigate the effect of N on the environment and promote a more efficient use of N in the rumen (Edwards, Parsons, Rasmussen, & Bryant, 2007; Lee et al., 2012; A. Stewart & Hayes, 2011).

Meanwhile persistency of cultivars in pastures depends, amongst other factors, on the capacity to maintain a high tiller density, and the ability to tolerate various stresses (A. Stewart & Hayes, 2011). The use of new strains (AR1 and AR37) of the endophytic fungus Epichloë festucae var. lolii (formerly Neotyphodium lolii; Leuchtmann, Bacon, Schardl, White, & Tadych, 2014) has contributed to the delivery of plants able to persist better under stress conditions created by insects, and with reduced or no toxicity to grazing animals (Hume, Ryan, Cooper, & Popay, 2007; Thom, Popay, Hume, & Fletcher, 2013).

In New Zealand, pasture grass testing started in the late 1920’s with the establishment of the Plant Research Station at Palmerston North (Hunt & Easton, 1989) and continued with the Department of Scientific and Industrial Research (DSIR) Grasslands Division (established in 1936). In the mid-1980s, the government reduced its participation in cultivar development, which was then taken up by private breeding companies (Hay & Lancashire, 1996). At the same time, and as a result of the same government policies (Lee et al., 2012), the compulsory cultivar testing scheme that had operated in New Zealand was abandoned, and in 1992, the Department of Scientific and Industrial Research (DSIR) was reconstituted into Crown Research institutes (Hay & Lancashire, 1996). Due to the need to

deliver cultivars with proven benefits, a voluntary testing system called National Forage Variety Trial® (NFVT) was developed by the New Zealand Plant Breeding and Research Association Inc. (NZPBRA) and trials started in 1991 (New Zealand Plant Breeding and Research Association Inc.) in co-operation with AgResearch Grasslands. Perennial ryegrass evaluation trials run for three years and three months and cultivars, must have been through a minimum of three trials within region in order to be included in NFVT yield summaries approved by NZPBRA.

In 2011, an initiative between DairyNZ and the NZPBRA was established: the 2011 Forage Review group. One of the recommendations of this review, was to finalise the Forage Value Index (FVI) (DairyNZ) which ranks perennial ryegrass (and short-term ryegrass) cultivars based on their relative economic benefit to pasture-based dairy systems (Chapman et al., 2012; Chapman et al., 2016; Chapman, Edwards, et al., 2015; Chapman et al., 2014), and in 2012 the FVI was launched. This index includes the key trait of seasonal DM yield for which performance values are calculated using data from the NFVT trials. Cultivars are then ranked for their estimated profit index in the FVI. A similar index was developed in Ireland as well, including the traits: spring, midseason, and autumn grass DM yield, grass quality, first- and second-cut silage DM yield and sward persistency (McEvoy, O’Donovan, & Shalloo, 2011).

NFVT trials are usually conducted using perennial ryegrass monocultures and in general, high N fertiliser inputs (3 % of mean dry matter harvested, Easton et al., 1997; Easton et al., 2001), while the standard farm practice in New Zealand is to sow perennial ryegrass in a mixture with white clover. In 2001, Easton et al. (2001) reviewing the results of 17 trials established between 1991 and 1996 throughout New Zealand, found that although the relative mean yield of some cultivars varied across regions (Canterbury and the North Island), yields were mostly consistent and no evidence of

interaction with management (pure grass or grass with clover) was detected. Nevertheless, management practices in New Zealand production systems have changed since the 1990s. An important increase in N fertiliser use during the decade 1991 – 2001 (MacLeod & Moller, 2006), the intensification of the dairy industry (Clark, 2011), the use of irrigation and the release of new perennial ryegrass and white clover cultivars have all created the conditions necessary for

reconsidering the interactions between these two species in a mixed sward and the implications that these possible interactions may have on the relative ranking of perennial ryegrass cultivars based on their herbage DM yield.