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

Abstract

Two studies are reported which investigated the validity of the theory of Plice (J. Range Mgmt. 5: 69. 1952) that decreased sugar in forages heavily fertilized with dung or N fertilizers (due to a P/N imbalance) causes the forage to become unpalatable to cattle. The 1st study revealed that heavy applications of P would not overcome unpalatability of dung-affected brome and that heavy applications of N as ammonium nitrate would not cause brome to become unpalatable, both situations contradictory to the proposed theory. The 2nd study confirmed the earlier results which showed the failure of N fertilizers (ammonium nitrate and urea sources) 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. While a lowering of ethanol-soluble sugar did occur in brome receiving these additives (as predicted in Plice''s theory), the fact that the N applications did not render brome unpalatable, while sheep manure did, completely refuted 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 cattle feces, urine and water (when no significant change in crude protein, P, or sugar occurred in the forage) further indicated that Plice''s theory is invalid.