A field study was made of the tiller dynamics of the New Zealand perennial ryegrass (Lolium perenne L.) cultivars Grasslands Ruanui, selected from a Hawke's Bay ecotype, and Ellett, selected from the Mangere ecotype. The work tested whether nitrogen (N) fertiliser or irrigation (Irr) could manipulate the persistence strategies of these morphologically distinct perennial ryegrass ecotypes, represented by these cultivars. Tiller births and deaths, and tiller survival, were monitored over 2 years, under rotational grazing by dairy cows. Patterns of cultivar tiller birth and death were similar. Much of the temporal change in tiller population density was shown to be a response to increasing or decreasing herbage mass associated with seasonal feed surplus or deficit. The probability of tiller survival between observation dates varied markedly with date, and was lowest in autumn. The cultivar effect on tiller survival was small but a significant (P < 0.05) cultivar × date interaction was detected, with survival probability for Ellett tending to be higher than for Grasslands Ruanui in March–April 1997 but lower from July 1997 until March 1998. A cohort effect on tiller survival was also identified, in that tillers formed in January, July, and August 1997 had lower survival probability than those formed at other times. N-treated plots produced 34% more tillers than did untreated plots, and irrigation had no effect on overall tiller density. N and Irr treatments had variable effects on tiller survival. Size density compensation theory and analysis of survival probability are introduced to aid the interpretation of tiller dynamics data.

Genetic improvement programs for livestock and pasture plants have been central to the development of the New Zealand (NZ) pastoral industry. Although genetic improvement of livestock is easily shown to improve animal production on-farm, the link between genetic improvement of pasture plants and animal production is less direct. For several reasons, gains in farm output arising from improved plant performance are more difficult to confirm than those arising from livestock improvement, which has led to some debate in the livestock industries about which plant traits to prioritise in future breeding programs to deliver the greatest benefit. This review investigates this situation, with the aim of understanding how genetic improvement of perennial ryegrass (Lolium perenne L.), the predominant pasture grass, may more directly contribute towards increased productivity in the NZ dairy industry. The review focuses on the dairy industry, since it is the largest contributor to the total value of NZ agricultural exports. Also, because rates of pasture renewal are greater in the dairy industry compared with the sheep and beef industries, genetic gain in pasture plants is likely to have the greatest impact if the correct plant traits are targeted. The review highlights that many aspects of ryegrass growth and ecology have been manipulated through breeding, with evidence to show that plant performance has been altered as a result. However, it is not clear to what extent these gains have contributed to the economic development of the NZ dairy industry. There are opportunities for breeders and scientists to work together more closely in defining economic traits that positively influence pasture performance and to translate this information to objectives for breeding programs, systematically linking information on the measured traits of ryegrass cultivars to economic values for those traits to assist farmer decision-making regarding the most appropriate cultivars to use in their farm system, and better defining genotype × environment interactions in key productivity traits of modern ryegrass cultivars. Changes in priorities for investment of public- and industry-good funds in forage improvement research and development will be needed if these opportunities are to be captured.

Perennial ryegrass (Lolium perenne L.) infected with a novel endophyte (AR37 or AR1), Wild‐type endophyte or no endophyte (Nil) was sown with white clover (Trifolium repens L.) in autumn 2005. The pastures were rotationally grazed by dairy cows from 2005–2009. Annual dry matter (DM) yield did not differ but AR37 pastures had a higher ryegrass tiller density, especially after the 2008 summer drought (+130%), and less white clover than did AR1 pastures. Concentrations of alkaloids produced by the Wild‐type association (lolitrem B, ergovaline) followed the same seasonal trends as did the AR37 alkaloids (epoxy‐janthitrems) but summer drought reduced concentrations of lolitrem B and epoxy‐janthitrems to less than half the mid‐summer (February) peak concentrations in the other years. Insect pests were monitored annually between 2006 and 2009. Tiller damage by Argentine stem weevil (Listronotus bonariensis (Kuschel)) was significantly reduced by all endophyte treatments. African black beetle (Heteronychus arator (F.)) populations in soil samples increased during the experiment with Nil > AR1 > Wild‐type = AR37. Root aphid (Aploneura lentisci (Pass.)) infestations followed the pattern AR1 > Nil > Wild‐type = AR37. A lower pest pressure from all insect pests in AR37 pastures is likely to have contributed to this treatment having the highest ryegrass tiller densities.

There is uncertainty among plant breeders as to which characteristics to select for to optimize grass growth dynamics. The objective was to study the relationship between leaf length and tillering in perennial ryegrass (Lolium perenne L.). Two long‐leaved and two short‐leaved genetic lines were grown in simulated shade and near‐full sunlight (control) environments. The genetic lines were New Zealand cultivars Ellett (long‐leaved) and Grasslands Ruanui (short‐leaved), both early flowering. The other two were late‐flowering divergent selections ‘LL’ (long‐leaved) and ‘SL’ (short‐leaved). Differences between genetic lines in leaf length were attributable mainly to higher leaf elongation rate (LER) in the two long‐leaved genetic lines, and leaf elongation duration (LED) did not differ significantly between genetic lines. Grasslands Ruanui had a higher tiller number per plant than Ellett in both light environments, explained by higher site filling but similar leaf appearance rate (AL). In contrast, LL had a higher tiller number per plant than SL, arising from a higher AL in LL. This difference decreased during the experiment under the control treatment because SL tended to have a higher site filling ratio than LL. However, in the shade treatment, differences in tiller number between LL and SL were more mediated by AL than site filling. Therefore, selection for high LER and long lamina length, even though associated with reduced site filling in all treatments, did not necessarily result in reduced tiller number per plant.

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