Our objective was to study in long-term polyfactorial experiments the effect of monoculture growing on winter wheat yields and to eliminate adverse effects of a high concentration of cereals through organic fertilization and interrupting crops. Field experiments established at Borovce belong to subregions Podunajska pahorkatina and the soil-ecological region Trnavska pahorkatina. Elevation of experimental site is 167 m, with average annual temperature of 9.2 degrees C and atmospheric precipitation sum 625 mm. The soil is degraded chernozem with humus content in topsoil 1.8 to 2.0%. Stationary experiments were established in 1975 by Blaha's method in four replications. Two winter wheat varieties, Livia (I) and Ilona (II) were included in these experiments. Evaluated crop sequences: 1. winter wheat monoculture with use organic fertilization; 2. rotation of winter wheat and spring barley with use of organic fertilization; 3. inclusion of interrupting crop maize for silage; 4. inclusion of interrupting crop oat. Variety: straw ploughing down (SL); straw ploughing down + green manuring (SL + Z). The lowest yields have been obtained with both varieties in winter wheat monoculture in that variant, where the straw ploughing down was applied with green manuring, reducing yields in Livia variety (I) by 0.10 t.ha(-1) and in Ilona variety (II) by 0.14 t.ha(-1) compared to control. Significant yield increase of varieties due to organic fertilization was in individual years. On the average of years, however, any form of organic fertilization did not influence positively wheat yield, on the contrast, it partly reduced the yields (Tab. I). In spring barley-winter wheat rotation and application of straw ploughing down with green manuring yield increase was found in both varieties due to organic fertilization depending on weather conditions in individual years. Significant yield increase of varieties resulting from organic fertilization in Livia variety (I) was 0.34 t.ha(-1) and in Ilona variety (II) 0.11 t.ha(-1). Yield reduction of winter wheat depending on duration of its growing was not found out (Tab. II), inclusion of maize for silage between two winter wheat varieties had a positive highly significant effect on yields (Tab. III) that varied, however, according to weather in individual years. A different response of varieties to forecrops has been found. Highest yield 9.09 t.ha(-1) was reached in 1992 with Ilona variety grown after direct forecrop maize for silage. On the average of the years the yield of Ilona variety grown after forecrop maize for silage was higher by 0.45 t.ha(-1). Similar results were obtained with oat as an interrupter of cereals sequence. In all experimental years and in both varieties (Tab. IV) higher yields of winter wheat were recorded after oat used as a direct forecrop. The increase ranged from 0.01 to 0.49 t.ha(-1) but it was not significant. Statistically significant differencessignificant. Statistically significant differences were found between years and varieties. Tn our experiments we have studied also the effect of organic fertilization and inclusion of interrupting crops on basic yield forming elements (spikes number per 1 m(2), grain number per spike, and thousand-grain weight). Between varieties and agrotechnical measures there were found differences in spikes number per 1 m(2) and grain number per spike. Thousand grain weight was the most stable element, ranging from 43.2 to 45.2 g in Livia variety and from 37.9 to 39.4 g in Ilona variety. Rotation of cereal species did not affect yield-forming elements studied. Greater differences in spikes number per 1 m(2) were found in variants with inclusion of interrupting crops (maize for silage and oat). When interrupting crops were included (maize for silage and oat), spikes number per 1 m(2) and thousand grain weight increased.
