Applications of paclobutrazol (or PP333, Cultar, Parlay; Syngenta, Basel, Switzerland) [(2RS,3RS)-1-(4-chlorophenyl)-4,4-dimethyl-2-(1H-1,2,4-triazol-1-yl)pentan-3-ol] at 2.0 kg ai/ha (= 8 L Cultar/ha) on heavy water-retentive soils in the Manawatu, New Zealand, when applied at spikelet initiation, shortened stems and resulted in seed yield increases of 300 to 350 kg/ha (Hare et al., 1990). At Lincoln, New Zealand on lighter soils and with late spring applications, no seed yield response to Cultar was observed. In these trials the PGR probably was applied too early for optimum response. In Oregon, paclobutrazol (trademarked as Parlay) resulted in results similar to those obtained in New Zealand. However, this compound was never registered for use on seed crops in either New Zealand or the United States because of soil persistence and the potential for residual effects on following crops.

In 1999 a new generation of PGRs became available. Trinexapac-ethyl [ethyl (RS)-4-cyclopropyl(hydroxy)methylene-3,5-dioxocyclohexanecarboxylate; trademarked as Moddus (Syngenta) in New Zealand and Palisade (Bayer) in the United States], a foliar-applied PGR that reduces growth through a reduction in plant levels of gibberellin, is registered for use in tall fescue seed crops in both countries. Responses have been variable among years, although generally positive. In Oregon, applications made to a turf tall fescue in 2000 and 2001 averaged 61 and 46% increases in seed yield, respectively (Silberstein et al., 2001a, 2002a), whereas a 21% yield increase occurred in a similar trial in 1999 (Silberstein et al., 2000a). Stem shortenings of between 13 and 45% have been reported for trinexapac-ethyl rates of 100 to 600 g ai/ha (Silberstein et al., 2000a). Recent data from Oregon and New Zealand showed that the best yields were achieved with 100 g ai/ha (400 mL/ha Moddus) applied from flag leaf emergence to early heading (see Appendix 23-2 and 23-3), although larger yield responses have been obtained at higher application rates in New Zealand with forage tall fescue. This probably is because forage tall fescue crops in New Zealand usually lodge before flowering, and Moddus prevents preflowering lodging (Fig. 23-4). Typical responses in New Zealand are illustrated in Fig. 23-5.


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Fig. 23-4. Lodging scores on 6 November (●), 30 November (■), and 17 December (▲), with associated regression curves and correlation coefficients, when forage tall fescue was sprayed with Moddus at several rates (Rolston et al., 2002b).


Fig. 23-5. Relative seed yield of Advance and ‘Vulcan' tall fescues at different Moddus rates (100 = no Moddus) (Rolston et al., 2002b).



Apogee (prohexadione-calcium, calcium 3-oxido-5-oxo-4-propionylcyclohex-3-enecarboxylate; BASF, Ludwigshafen, Germany) has resulted in responses similar to those from Moddus in Oregon, when the means of a range of rates (Table 23-9) or similar rates (Table 23-10) were compared. In the only New Zealand trial with a forage tall fescue, Apogee appeared to be more active than Moddus for the same rates (Rolston et al., 2002a,b). In Oregon, Moddus and Apogee have given similar responses at similar active ingredient rates (Tables 23-9 and 23-10). In New Zealand, using a different formulation of Apogee, the seed yield response of Apogee was slightly larger than for Moddus (Table 23-11).


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Table 23-9. Relative seed yield of Velocity tall fescue in Oregon in response to plant growth regulators (PGR) (data derived from Silberstein et al., 2001a,b; 2002a,b).

Table 23-10. Seed yield and relative seed yield of Velocity tall fescue in response to plant growth regulators (PRG) in Oregon (Silberstein et al. (2001a,b; 2002a,b).

Table 23-11. Effects on seed yield of ammonium sulfate (AMS) at 0.5 kg/ha as an additive to Moddus (Rolston et al., 2002a).


The addition of ammonium sulfate (AMS) to Apogee did not appear to improve its activity (Silberstein et al., 2002b). A similar trial with Moddus also was inconclusive about the impact of AMS, since there was no response at 800 mL/ha, but there was a significant effect at 1600 mL/ha (Table 23-11).

When there was only a 20% seed yield increase due to Apogee application, there was no difference between two application dates 10 d apart (Silberstein et al., 2000a). In another trial (Silberstein et al., 2001a) with a 60% seed yield increase, Moddus resulted in seed yield responses with three application dates at different stages of development [node-flag leaf emergence, early heading (+6 d), or full heading (+18 d after first application)]. When flag leaf emergence was compared with early heading, 12 d later, there was no difference in seed yield from applications of Apogee (Silberstein et al., 2001b).

In a number of tests with Moddus or Apogee, there was no difference in seed yield when single applications were compared with split applications (Gingrich and Mellbye, 2001; Silberstein et al., 2000a,b; 2001b; 2002b; Rolston et al., 2002b).

Few trials have comparative PGR data between stands of different ages, especially comparing first-year stands with older stands. Good yield responses (69% seed yield increase from 600 mL/ha) were reported for a first-year stand in Oregon (Silberstein et al., 2001a).


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