Pubdate: October, 1956:
Source: Fibres (Engineering and Chemistry)
Author: Carl V. Feaster, Agronomist, Field Crops Research Branch, Agricultural Research Service, U.S.D.A.

MONOECIOUS HEMP BREEDING IN THE UNITED STATES

The hemp breeding programme in the United States is being directed toward improvement of fibre quality through the development of strains with uniform maturity among plants. Present commercial varieties are dioecious, with the male plants returning about three weeks before the female plants. This results in fibre of different maturity and consequently less uniformity.

The development of monoecious strains, where all plants mature at approximately the same time, is an effective means of eliminating much of the variability in fibre quality among plants. Monoecious hemp also may allow for changes in processing methods to the extent that water retting would be economically feasible. At present, hemp in the United States is dew retted.

Trials and results

The original monoecious plants were selected by Borthwick and Scully*, while conducting photoperiodic studies with Kentucky hemp, a dioecious variety. This material was released to the Hemp Improvement Project and a monoecious hemp has been developed by several generations of sibbing selected monoecious plants. The monoecious character has become relatively stable. A few female plants appear in some strains; however, no male plants occur unless foreign pollen is involved.

Yield trials comparing Kentucky hemp with Kentucky monoecious show the yield of fibre to be about the same for the two varieties; however, the percentage of fibre has averaged slightly higher for the dioecious variety and the stalk yield slightly higher for the monoecious variety.

Evaluation of individual plants from Kentucky monoecious showed considerable variation in the percentage of fibre, stem weight, stem weight, stem diameter, and weight/diameter ratio of the stem. Per cent fibre was negatively correlated with the other measured characters. That is, plants with low stem weight, small stem diameter, or weight/diameter ratio of the stem were relatively high in per cent fibre. When the progenies from these selected selfed plants were grown, heritability was found to be high for per cent fibre but was negligible for stem weight, stem diameter, and weight/diameter ratio of the stem. These results indicate that although stem weight, stem diameter, and weight/diameter ratio of the stem is correlated with per cent fibre, the stem measurements do not express the inherited phase of per cent fibre. The measurements do, however, account for some of the differences in per cent fibre which are due to environment. When proper adjustment of per cent fibre was made on the basis of stem weight, stem diameter, or weight/diameter ratio of the stem, the variability due to genetic differences was more evident and selection of plants inherently high in fibre was more effective.

The genetic variability and heritability of a character is indicative of the extent to which improvement is possible through selection. Improvement of fibre content within this strain appears promising since it is relatively heterozygous for per cent fibre and heritability of per cent fibre is high.

Some of the details of the above work appear in an article entitled 'Genetic and Environmental Variability of Percent Fibre and Other Characters in Monoecious Hemp, Cannabis Sativa L.' The Textile Quarterly, Volume 6, No. 1

*Borthwick, A., and Scully, N.J. Photoperiodic Responses of Hemp. The Botanical Gazette 116; 14-29. September 1954