Seed and soil preparation techniques influence on the establishment and growth of three common subtropical pasture species

Abstract

It is often a challenge to successfully establish perennial pastures which in turn determines the production potential and persistence thereof. Three perennial subtropical grass species commonly cultivated in South Africa are Eragrostis curvula (var. Ermelo), Digitaria eriantha (var. Irene) and Chloris gayana (var. Katambora). The secret of successful planted pastures lies in the execution of good seed- and soil preparation techniques; also referred to as pre-establishment techniques. These techniques include: seed conditioning by coating seed with either conventional coating or a coating that contains Mycortex, that differs in only adding mycorhizza; adjusting the seeding rates recommended by industry which potentially causes intra-species competition (competition between plants of the same species); planting an annual nurse crop with a perennial species which potentially causes inter-species competition (competition between plants of different species); and controlling seedbed qualities before planting and consolidating the seedbed after planting. There were four experiments conducted (Randomized Block Design) in a controlled environment of a Phytotron. In these experiments the growth factors like water, temperature and soil variance were controlled. It must be kept in mind that conclusions made from data obtained in these experiments cannot be extrapolated to field conditions. Digitaria eriantha and C. gayana were only assessed in two small experiments as uncoated and coated (conventionally- and Mycortex coated) seed. Eragrostis curvula was measured in two experiments as uncoated and conventionally coated seed and was also evaluated at different seeding rates, planted with and without an annual nurse crop (Eragrostis tef) and where seedbed consolidation was done after planting. The treatments were replicated between three and six times in the four experiments. It was found that coated seed does not significantly improve establishment of any of the test species. Adapting the E. curvula seeding rate had no significant effect because of the favourable environment created. Consolidating the seedbed caused a lower emergence and yield of E. curvula as a result of an anaerobic condition being generated around the seed. The nurse crop planted with E. curvula caused an increased yield in the first season. Pre-establishment techniques applied as treatments in these controlled experiments were not as effective as hypothesized in a controlled environment. There were three field experiments conducted (Randomized Block Design) in uncontrolled environments. Establishment success of test species were evaluated as a response to variable seeding rate treatments, of uncoated and conventionally coated seed; additionally, D. eriantha and C. gayana Mycortex coated seed was also used. The establishment success of the perennial test species was also measured as being influenced by an annual nurse crop (Eragrostis tef). The experiment included the evaluation of establishment success of E. curvula where seedbed quality was controlled through preparation techniques; rotavating before planting and consolidating the seedbed after planting. It was concluded that a higher seeding rate gives a better first season plant density. However, in season two, the optimum plant density was reached for E. curvula and C. gayana and there were no differences between seeding rate treatments. This was as a result of intra-species competition in season one, resulting in plants out-competing one another. The creeping growth characteristics of C. gayana also contributed to this result. One of the main functions of the nurse crop planted with a perennial grass species is to create a more “favourable” competition as opposed to aggressive competition from weeds. The establishment success of perennial species was lower, where nurse crops were included as a result of inter-species competition. From the second season data it can be seen that D. eriantha and C. gayana did not have lower plant densities where a nurse crop was planted, whereas E. curvula still had lower plant densities. All species had a significantly higher pasture density in season one where uncoated seed was used due to a higher number of seed planted at the same seeding rates on a weight basis. Nevertheless, in season two, the E. curvula and C. gayana plant densities showed no significant differences between the seed conditioning treatments. This is attributed to the small seed size and shape of E. curvula and the creeping growth characteristics of C. gayana. Controlling the seedbed quality was found to be unsuccessful for E. curvula. The field experiment results highlight the interactive effects between seed coatings, seeding rates and nurse crop treatments as a function of selected species. The insignificant results obtained from the controlled environment study support the original hypothesis that favourable growing conditions reduce the functions of pre-establishment techniques during the facilitated establishment of sown pasture seed. The significant results from the unfavourable field study support the original hypothesis that adjusting the seeding rates and the use of a nurse crop does have significant effects on the establishment success of the perennial species. It also confirms that conditioning of seed by using coating substances does have significant effects on the establishment success of the perennial species. Controlling the seedbed quality does not have significant effects where pastures are irrigated until establishment.