Innovative breeding of high-digestibility kikuyu cultivators to increase milk production

Summary

Kikuyu grass is a highly productive forage species with only intennediate digestibility. It is probably the second most important grass for the Australian dairy industry (after ryegrass) but has received comparatively little breeding attention. The gennplasm base of kikuyu in Australian agriculture is certainly narrow - most likely, dangerously so. There is currently no kikuyu breeding program operating in Australia and earlier work consisted primarily of selection from very limited introductions. Milk production would be significantly increased if a kikuyu cultivar with improved digestibility were available.

One method of increasing disgestibility is to lower the lignin content of the plant (lignin is an indigestible fibre component of all plants). This has been achieved in maize, sorghum and millet by the production of so-called 'brown mid-rib' types. The brown mid-rib type is caused by a single gene mutation which results in lower lignin content, and results in a characteristic brown-coloured mid-vein on the undersides ofleaves. There is no gennplasm collection available for kikuyu and no brown mid-rib types are known. The aim ofthis project was to produce a brown mid-rib cultivar and, thereby, raise the digestibility of kikuyu pastures and their value to the dairy industry. Kikuyu digestibility needs to be raised by about 10 percentage points to be comparable with ryegrass. Two chemicals were used (singly and in combination) to treat seeds ofthe kikuyu cultivar Whittet. The chemicals were chosen to cause genetic damage to the kikuyu hereditary material (DNA) and result in new genotypes (mutants). It was hoped that the new genotypes would include either a brown mid-rib type, or some other mutant that resulted in increased digestibility.

This mutagenic approach was a high-risk strategy compared to conventional plant breeding but was relatively low-cost and rapid. There was a small probability oftotal success using this 'hit and-miss' method and for certain progress over the long-tenn a full, conventional breeding program for kikuyu would need to be funded. Forty thousand single plants of kikuyu were grown in the field under irrigation for two years. In the second year, repeated visual selection was used to identify 212 unusual-looking individuals. In addition, a number of random selections were made as controls, bringing the total to 318. These plants were transplanted to the glasshouse, and 30-day leaf regrowth was harvested. The leaf samples were analysed for digestibility in the laboratory using the sheep rumen-fluid method. Seventy four plants were identified which had a digestibility greater than 73 %, six of which were greater than 76%. This is a marked improvement over nonnal kikuyu, which is usually around 65-68% digestible under field conditions, and averaged 71 % in the untreated glasshouse controls in this project. These promising lines require further testing prior to their release to industry.

Residual seed from this work was sent to Wollongbar where (outside the contract for this project) one thousand individual plants were screened in the glasshouse for resistance to kikuyu yellows disease. Yellows is becoming a serious problem, particularly in older coastal pastures,and there is no cheap, effective control option. Therefore, genetic resistance to yellows would be a valuable character in any new kikuyu cultivar. One kikuyu plant has survived three challenges from the disease and appears to be resistant - five others have survived two tests. These lines too require further testing with a view to releasing a new cultivar.

Given the promising results to date, DRDC and NSW Agriculture need to negotiate the funding of further glasshouse and field experiments to establish the value ofthe lines produced in this project. Potentially they could have a very significant effect on the Australian dairy industry. DRDC and its research providors should discuss the need for a kikuyu germplasm collection and funding for a kikuyu breeding pro gam in Australia.