A dynamic model for determining the optimum cutting schedule of Italian ryegrass

POHJONEN, V. 1975. A dynamic model for determining the optimum cutting schedule of Italian ryegrass. J. Scient. Agric. Soc. Finl. 47: 71-137.

Abstract. A quantitative approach to the determination of the optimum cutting schedule of Italian ryegrass (Lolium multiflorum Lam.) was presented. Optimization was based on a dynamic growth model which included the concepts crop growth rate, development rate and proper growth rate. The proper growth rate measured the growth potential associated with the stages of development in the sward.

The crop growth rate of Italian ryegrass was studied at the Arctic Circle Experiment Station during 1973 and 1974. The proper growth rate was determined from primary observations as the derivative of an ordinary logistic curve which passes through origin. The maximum theoretical daily growth of Italian ryegrass was calculated as approx. 300 kg/ha/day.

The optimum cutting schedules using gradient method were sought for Italian ryegrass sward. First, the maximum total dry-matter yield was looked for. Then the maximization was extended to the case when the yield was weighted with the digestibility of the dry-matter. The maximum yield was obtained when the sward was cut three times, and when the cuttings were concentrated into the latter half of the growing season.

The yield of the optimum cutting schedule was not sensitive to small changes in the cutting dates. In the conditions of Finnish Lapland the optimum cutting schedule of Italian ryegrass was: first cut at the end of July, second cut at the end of August and the third cut at the end of September.

1. Introduction

Apart from some abiotic and biotic environmental factors such as fertilization, the water balance of the soil and the density of the ley, the size and quality of the annual yield of sown grassland can also be affected by the dates chosen for cutting. The dates can be selected within the growing season for example so that dry-matter production is maintained at as high a level as possible. In order to ensure that as much as possible of the incoming radiation is bound during the growing season, the cutting dates should be arranged according to a specific plan (COOPER 1966).

The optimum cutting schedule for a silage ley can be defined as the choosing the cutting dates within the growing season such that the total yield obtained as the sum of the separate cuttings is maximized. For example, the total dry-matter yield itself can be maximized. However, this is not sufficient since in grassland management another form of output is regarded as being more useful. For example it is possible to weight the dry-matter yield with the crude protein …

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