SUMMARY
The agronomic value of legume grass associations and performances under grazing and drought conditions have recently presented great interest for two reasons: firstly, because the above-mentioned associations are still under discussion from the agronomic point of view, and secondly, due to the increase of global warming, the establishment of legume grass associations would be considered as a new issue concerning more forage biomass production under drought conditions. A series of growth analysis parameters were measured in the field on irrigated and non-irrigated plants of Medicago sativa v. Yliki and Festuca arundinacea cv festorina in pure (monoculture) and mixture (1:1 associations) stands. The plants were cut four times (3/5, 23/5, 17/5, 9/7) during the period of growth in 1997. The results suggest that in 1:1 associations, a change in the nature of the interference effects from competition to neutralism takes place as we pass from irrigated to non-irrigated treatment. In this situation of neutralism the non-irrigated plants of M. sativa present higher “relative yields” than the irrigated, although in this association (1:1) the biomass production (DM g/m2) of M. sativa is lower than the one obtained from the pure stand.
Keywords: alfalfa - tall fescue association, interference, plastic response, relative yield.
INTRODUCTION
Forage legume grass mixtures are important components of animal production systems. The greatest potential benefits of grass legume associations, compared to pure legumes are: (i) increased soil and water conservation; (ii) enhanced weed control; (iii) reduced risk of total stand loss due to thinning of legume stands; and (iv) bloat protection for animals grazing exclusively legumes (Casler and Walgenbach, 1990). However, grass species are highly competitive to legumes, especially to alfalfa (Medicago sativa L.) plants (Smith et al., 1992).
Today there is a lot of questioning about the usefulness on the establishment of alfalfa grass mixed cropping systems. Although there is evidence from a lot of experiments for total yield shifting (Chamblee and Colins, 1988), in such mixed practices, the value of alfalfa-grass associations is still under consideration, not only from the agronomic point of view, but also from the stand of the nature of the interference effects produced within the associations themselves (Zannone et al., 1986). However, perennial grasses, especially when they form mixtures with medics, could potentially be used to promote the exploitation of fallow lands. Also, this practice is suggested as an alternative one, especially for lands which were previously cultivated by cereals and tobacco (Vaitsis and Karamaligas, 1996). The objectives of this study were to investigate the response of the alfalfa (Medicago sativa v. Yliki), in absolute and relative yield terms, when it forms binary mixtures with tall fescue plants (Festuca arundinacea cv festorina) and to screen domination vs. vigour as it reflects the association effects. The plastic responses (Jacquard, 1972), as they express the nature of the interference effects, were also measured. The responses of the component species to cutting and drought conditions were also taken into account.
MATERIAL AND METHODS
The experiment was conducted under the same soil conditions in the field area of the Tobacco Institute of Drama, Northern Greece. Its latitude is 41°09' and altitude 130 m above sea level. The mean annual temperature is 15.2°C and the total annual precipitation is 616 mm. The used species were Medicago sativa v. Yliki, which was produced at the Institute of Forages of Larisa and Festuca arundinacea cv festorina, introduced from the USA.
Pure and mixed (1:1) field plots (1 x 1 m), completely randomized, with a sowing density of 4.5 g/m2 were established in autumn 1995. All stands were irrigated until the plants were well established. After the first cut in spring 1996, no water was applied on half of the plots (H0). The rest were well irrigated by sprinklers to maintain field capacity (H1). The plants of the half plots were cut (at 7 cm from the soil surface) four times during the growing season (at approximately 20 day intervals), while the rest were cut at the same height all at once, as they achieved full maturity. The plants were oven dried at 75°C for 48 hours and the above ground biomass (g/m2) was determined. There were three replicates for each treatment of pure stands and four for mixed ones.
The Maitre proportional model (de Wit 1960, Maitre, 1977) was applied to cut by cut data, in both irrigation treatments, in order to define the importance of the 1:1 binary mixture vs. monoculture, in relative yield terms, per species:
Yij/Yii (relative yield of species i), Yji/Yjj (relative yield of species j) (1)
where Yij, Yji, Yii and Yjj are the above ground biomass (g/m2) of species i and j in mixed and pure stands, respectively.
The nature of interference effects was expressed by the “plastic responses”, as defined by Jaquard (1972) and calculated by the formula:
[n(Yij/Yii)]-1 (2)
where n is a multiplication factor relating the population in mixture to the same unit area of monoculture and in our case equals 2 (Zannone et al., 1986).
The relative domination (RD) was calculated by the formula:
RD= Yij/Yji (3)
and the vigour ratio (VR) by the formula:
VR= Yii/Yjj (4),
as suggested by Zannone et al. (1983).
RESULTS AND DISCUSSION
In the cut by cut sequence, regardless of the watering regime, alfalfa plants growing in pure stand resulted in higher absolute yield values than in mixed stand (Figure 1). Due to the lower alfalfa yield values, provided by the mixed stand, the relative yield was always less treatment, as a result than 1 (Figure 2). These values started progressively to increase in H0 of the relative earliness that tall fescue plants appear in relation to the alfalfa plants. When the available water is in depletion, then the tall fescue plants, owing to the earliness of their root formation (soft-seeded plants) and their tuft root system, exhibited higher and more efficient exploitation of the soil's surface water table than alfalfa (hard-seeded, woody root formation).
Figure 1. Cut by cut yield changes of alfalfa, obtained from pure and mixed stands, for both irrigation treatments.
Figure 2. Cut by cut relative yield changes of alfalfa, for both irrigation treatments H1,Ho. The lines parallel to X axis correspond to relative yields obtained when plants are all cut at once.
Obviously, the relative yield of alfalfa in the cut by cut sequences were higher in H0 treatment than H1. On the contrary, when the crop harvested all at once (17/6) the relative treatment. Probably, when the available water is in excess, yield of alfalfa was higher in H1 the alfalfa plants overcome the time delaying of their root formation, in respect to the tall fescue plants, resulting in higher absolute and relative yield values.
The seasonal plastic responses patterns are illustrated in Figure 3. Positive values for both species, obtained by formula 2, indicate cooperation, while negative opposition. Values close to 0, indicate neutralist interference effects. When species i responds to positive values while j in negative ones, there is strong evidence that species i competes to species j for the specific depleted resource (Zannone et al., 1986). Thus, in H1 treatment (Figure 3a) the seasonal plastic responses patterns illustrate the existence of a scaled and strongly asymmetric competition that the tall fescue plants appear to have on alfalfa plants, at least at the first three cuts. These patterns represent the water cumulative effects upon the competitive behaviour of the constitute plants of the mixture. When these responses are measured at the stage of fruit formation (fourth clipping), the results suggest neutralism, a fact that probably due to the compensational contribution that alfalfa fruits have, in yield terms. Conversely, in H0 treatment (Figure 3b), the cut by cut sequence resulted in neutralism. Thus, when water is in excess the tall fescue strongly competes with alfalfa, while when it is in depletion, this competitive effect gradually turns to neutralism.
The proportionality relationship that exists between the relative domination and the vigour ratio was also examined in order to illustrate the association effects that the mixed stand appears to have over the pure one (Zannone et al., 1983). The relationships between associating (or dominating) ability and pure stand performance (formulae 3 and 4) provide indications on the possibility of predicting the associating performance of plants (Zannone et al., 1986). This relationship seems to exist between the relative domination in the associations and the ratios of population vigour in the pure stands, where the associations effects are presented (Figure 4) considering alfalfa as the reference species. In H1 treatment negative association effects exist between the two species, as the correspond line is located under the isocline (competition), while in H0 the association effects led to neutralism, as the line is located over. The differentiation of watering regime, from H1 to H0, resulted in higher RD values, while the VR values are kept rather constant. So the association performance in H0 treatment, was more sensitive in the water effect than the pure one, but the simultaneous over-compensation effect of the pure stand (lower RD values than VR) led the association to neutralism. In the excess of the water (H1 treatment) there is no such over-compensation effect and further competition is established (Figure 4).
Figure 3A. Plastic responses of alfalfa and tall fescue in H1 treatment.
Figure 3B.Plastic responses of alfalfa and tall fescue in Ho treatment.
Figure 4. Association effects on alfalfa plants, obtained in Ho and H1 treatments, for cut by cut sequences.
The results suggest that though in absolute yield terms, the negative interspecific effect of tall fescue plants on alfalfa is the rule, a remarkably plasticity occurred when these results are examined in relative yield terms. Thus, in water depleted situations (H0 treatment), this plasticity is expressed as neutralism and when water is in excess (H1 treatment), as competition.
REFERENCES
Casler, M.D. and Walgenbach, R.P. 1990. Ground cover potential of forage grass cultivars mixed with alfalfa at divergent locations. Crop Sci. 30:825-831.
Chamblee, D.S. and Colins, M. 1988. Relationships with other species in a mixture. p. 439-461. In Hanson, A. A. et al. (Eds) Alfalfa and alfalfa improvement. Agronomy Monogr. 29, ASA, Madison, WI.
Jacquard, P. 1972. Glossaire des termes et definitions. Sect. Plantes Fourrageres EUCARPIA, CEPE, Montpellier, 36 p.
Maitre, J.P. 1977. Aptitude a la vie en association chez de Trefle violet. Ann. Amelior. Plantes, 27(3):369-387.
Smith, R.S. Jr., Bouton, J.H. and Hoveland, C.S. 1992. Persistence of alfalfa under continuous grazing in pure stands and in mixture with tall fescue. Crop Sci. 32:1259-1264.
Vaitsis, Th. & Karamaligas, Ch. 1996. Collection, selection and evaluation of tall fescue (F. arundinacea) germplasm. (In Greek with English Summary). In Proc. of the 6th Panhellenic Conference of Plant Improvement and Problems of the Modern Agriculture. Florina, 2-6 October 1996. Greece.
Wit de, C.T. 1960. On competition. Agric. Res. Rep. Verslag. Landbouwk. Onderzoek. 66:1-82.
Zannone, L., Assemat, L., Rotili, P. & Jacquard, P. 1983. An experimental study of intraspecific competition within several forage crops. Agronomie 3(5):451-459.
Zannone, L., Rotili, P., Paoletti, R. & Scotti, C. 1986. Experimental studies of grass-legume associations. Agronomie 6(10):931-940.
|
[26] Technological
Education Institute of Forestry, 66100 Drama, Greece [27] Dept. of Range Science, Aristotle University of Thessaloniki, P.O. Box 236, 54006, Thessaloniki, Greece [28] Dept. of Range Science, Aristotle University of Thessaloniki, P.O. Box 236, 54006, Thessaloniki, Greece |
