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Selective Grazing Induced by Animal Excreta. II. Investigation of a Causal Theory¹

Crops Research Division, USDA, Department of Agronomy and Plant Genetics

J. D. Donker

Department of Dairy Husbandry, University of Minnesota, Saint Paul

ABSTRACT

Two studies are reported which investigated the theory of Plice (14, 15), that decreased sugar in forages heavily fertilized with dung or N fertilizers caused the forage to become unpalatable to cattle. The first study revealed that heavy applications of P would not overcome unpalatability of dung-affected brome and that heavy applications of N would not cause brome to become unpalatable, both being contradictory to the proposed theory.

The second study confirmed earlier results which showed the failure of N fertilizers to render brome unpalatable. Similar increases in P in brome heavily fertilized with N or sheep manure accompanied increased crude protein with these treatments and dispelled the idea that a P/N imbalance occurs in plants fertilized in this manner. Although less ethanol-soluble sugar did occur in brome receiving these additives (as predicted in Plice's theory), N applications did not render brome unpalatable, whereas sheep manure did, refuting the claim that lowered sugar concentration in the plant is responsible for the unpalatability of dung-affected forage. The unpalatability of brome growing on plots treated with a dilute mixture of feces, urine, and water, which failed to cause significant changes in chemical composition, further indicated that Plice's theory is invalid.

¹ Paper No. 5380, Scientific Journal Series, Minnesota Agricultural Experiment Station, St. Paul, Minnesota.