166 C. R. BARNICOAT 



by their appearance and productivity, are healthy and thriving. 

 Excessive wear is not found when sheep are carried on low-produc- 

 ing, wiry native grasses (Table I). \\'ear does not appear to be 

 caused by simple abrasion; e.g., no undue wear is observed in 

 mouths of sheep maintained under hard conditions on sandy or 

 pumice country, although these abrasive siliceous materials are 

 splashed by rain and carried by wind onto the pasture plants. 



The appearance of worn teeth and the type of wear indicate that 

 the wear is caused by both erosive and physical effects. The erosive 

 action was first believed to be brought about by compounds in the 

 herbage sap complexing the calcium of the apatite (essentially tri- 

 calcium phosphate) which constitutes the inorganic, and major, 

 fraction of the tooth substance. In high-yielding pasture, chelating 

 agents such as acids of the Krebs cycle, amino acids, and phos- 

 phorylated compounds of carbohydrate metabolism would occur in 

 increased concentrations, especially in the meristematic tissues of 

 the actively metabolizing young leaves. 



Removal of the superficial lavers of the incisal surface of the 

 tooth could be further facilitated by the mechanical action of the 

 harder constituents of grass, such as the fiber, which at each bite is 

 pulled and stretched between the incisal surface and the pad of the 

 upper jaw before being nipped. "Improved" pastures are usually 

 rich in perennial rve grass (Loliiim perenne), which is generally 

 regarded as particularly destructive to sheep's teeth. Mechanical 

 tests also show that rye grass is considerably tougher than others, 

 especially native grasses. Baker et al. ( 1959), in Australia, concluded 

 that the opal phytoliths in plants could play an important part in 

 the abrasion of sheep's teeth. 



In order to study the effect of organic acids, polished sections of 

 air-dried sheep's teeth were exposed to 0.05 m solutions of organic 

 acids, pH 6.1, for 20 hours at 10°C, these concentrations of acid 

 and pH being similar to those of herbage press juice. Hardness tests 

 were made with the Tukon Hardness Tester before and after treat- 

 ment. Results are recorded in Table IV. These results indicate that 

 in solutions similar in properties to grass press juice the four acids 

 of the Krebs cycle readily attack teeth. Malic acid, the predominant 

 organic acid in grass sap, is even more active than citric, used in 



