Maize endosperm mutants affecting soluble carbohydrate content as potential additives in preparing silage from high protein forages

High protein forages, like alfalfa, have long been recognized as marginal with respect to the carbohydrate supply underlying the fermentation process leading to the prompt formation of lactic acid on which preservation, palatability, and nutritional value of high protein silages are dependent. An exploratory study was made of maize endosperm mutants as potential additives to the chopped, high protein forage at ensiling time as a protection against this deficiency. The criterion applied in evaluating mutants for this special purpose, in addition to suitability for production as a farm crop for local use, was content of sugars plus phytoglycogen (a highly branched, water-soluble polysaccharide) in the mature seed. Nine loci were brought under review, namely amylose extender (ae), brittle (bt), brittle-2 (bt2), dull (du), shrunken-2 (sh2), sugary (su), sugary-2 (su2), waxy (wx), and sugary enhancer (se). Attention came to focus on the sugary locus (su), which is well known as the site of the mutant underlying common sweet corn. About one-half the dry matter in mature su kernels is soluble carbohydrate (sugars plus phytoglycogen), a value about 10 times that for dent corn. The su mutant, however, is deemed impractical for use as a high protein forage additive in spite of its very high content of soluble carbohydrates. A basic finding in the present study is that there is a numerically close and inseparable, inverse relationship between soluble carbohydrate content and rate of water loss from the fully developed seed. Because drydown is a major factor affecting suitability of a strain for machine harvesting and subsequent processing of the grain, this adverse relationship imposes an upper limit on the soluble carbohydrate value that would be generally acceptable in a strain for local use as a high protein forage additive. The su mutant present in common sweet corn clearly exceeds this acceptable limit. A compromise, therefore, between the two opposing variables, high soluble carbohydrate content and drydown, was sought. Experimental data were obtained which indicate that an allele termed sugary-Brawn (su-Bn2) will meet the closely coupled, double requirement. The su-Bn2 mutant yields well, gives an approximately five-fold increase in soluble carbohydrates over dent corn, and markedly increases drydown as compared with su. Field observations suggest that harvestability of su-Bn2 would be made dependable if the mutant were incorporated in strains whose silking dates were about 10 days earlier than characteristic for adapted dent corn. A second intermediate mutant at the su locus, termed sugary crown (su-cr), was found to contain about three times as much soluble carbohydrate as dent corn and is somewhat more rapid in drydown than su-Bn2. These high soluble carbohydrate mutants are visualized for use as additives in the form of ground, dry grain, incorporated in the freshly chopped forage at ensiling time. Silage prepared from snapped ear shoots with green husks and containing mature, high moisture kernels is an optional form of additives that should be further tested.

R. A. Brink

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