Influence of age and year of growth on the botanical composition and productivity of swards

Abstract

SUMMARYAn account is given of vegetational and yield changes occurring in sown swards of Lolium perenne L. to which four alternative management treatments were applied for 8 years.With infrequent cutting (four cuts per year), and a fertilizer nitrogen rate of 188 kg/ha per year, there was a substantial invasion by Agrostis spp. At 376 kg N/ha under cutting the main invader was Agropyron repens (L.) Beauv. with a lower proportion of Agrostis. After cutting at 752 kg N/ha, the swards became dominated by A. repens. With rotational sheep grazing, at 376 kg N/ha, the main invader was Poa annua L., although the proportion was low and L. perenne remained the dominant species. The plant successions which occurred with each type of management as the swards became older had little influence on the yield or quality of herbage under grazing or cutting.Because sward age is confounded with the influence of year-to-year environmental variation, the experiment was designed to enable independent assessments of these factors to be made. Sequential sowings were made to produce an array of swards between 1 and 7 years old. Year-to-year fluctuations in the pattern of herbage production were quantified after allowing for the progressive increase in sward age. Similarly, the influence of age was investigated by adjusting yields for year effects, and comparing swards of different ages in the same year.Year-to-year variation in the total yield of dry matter was much greater than variation attributable to sward age. There was similarly more variation due to year than to age in the yield of D.M. at any one season. In the most extreme comparison, the D.M. yield at an individual defoliation in one year was 2·8 times the lowest yield at the same season in a different year, whereas the yield at the same season varied by a factor of up to 1·5 between swards of different ages.The influence of age on quality was quantified for cut swards only. There was an age effect only at the first cut, when the percentage of digestible organic matter (DOM) declined with age, and herbage nitrogen concentration increased, at the lowest level of fertilizer N, but these changes did not occur at higher N levels. Annual yields of DOM declined significantly with age, but only at the highest N level, and there were no significant changes in the annual yield of herbage N.Comparisons were made of the cumulative D.M. yields for several years of young swards, resown every 3 years, and undisturbed swards, up to 7 years old. Young swards gave a higher yield during the first two years after sowing, but the advantage was lost in the period of ploughing and establishment, during which time the undisturbed swards achieved a small advantage in cumulative yield. Cumulative yields of DOM and N from young swards were not sufficiently different from those of older swards to suggest that there would be an advantage from regular resowing. A theoretical analysis was made to predict the long-term yields of D.M. and DOM from different renovation strategies at yearly steps from 2 to 7 years. This suggested that ploughing and resowing gave little or no advantage in productivity over swards which were not renovated before age 7, and resowing too frequently gave a loss of yield.The paper contains an appendix by A. Smith and R. E. Deriaz describing a method used for the correction of analytical results obtained for variable contamination of herbage samples by extraneous materials.