626 RATE OF GROWTH OF DAIRY COW 



A comparison of Fig. 1, the theoretical velocity curve of an auto- 

 catalytic monomolecular reaction, with Figs. 2 and 3, the velocity 

 curves of growth of a group of dairy cows in this Station,^ shows 

 obvious similarities. 



These postuterine velocity curves of growth show two reactions or 

 cycles with maxima at about 5 and 20 months of age. The assyme- 

 try of the curve of cycle A may be tentatively explained by assuming 

 that the second cycle, B, begins before the completion of cycle A, 

 thus resulting in a superimposition of the two cycles with the con- 

 sequent assymetry. 



Other factors may conceivably enter, such as deposition of fat at 

 that age, as suggested by Robertson in connection with his work.^ 

 The other irregularities might be expected considering that the animals 

 were kept under average, which means highly variable, conditions 

 and considering the small number of animals on which the data are 

 based (Table I) . The irregularities in the second extrauterine cycle 

 B may be further accounted for by the fact that for reasons of econ- 

 omy the animals were bred at 20 to 21 months of age, calving at the 

 ages of 29 to 30 months. 



Cycle B is undoubtedly the last growth cycle, comparatively little 

 growth being made after the age of 30 months. According to Donald- 

 son^ and Robertson,^ there are at least three such cycles in the mam- 

 mal; if the tricyclic theory is true, then there should be at least one 

 cycle in utero in addition to the extrauterine cycles A and B. There 

 is some evidence in this department, not enough, however, for pub- 

 lication, indicating that there is indeed such an intrauterine cycle. 



Each of these cycles, according to Robertson,^ should follow the 

 equation of an autocatalytic reaction. It is interesting, in addition 

 to the qualitative comparison of the experimental and theoretical 

 curves, also to find out what is the approximate quantitative agree- 

 ment between the experimental and calculated values. Equation (1) 

 cannot be used for this purpose since, while the weights of the ani- 

 mals were recorded with reference to time, their age, the velocity 



° Eckles, C. H., Univ. Missouri Agric. Exp. Station, Research Bull. 36, 1920. 

 ^ Donaldson, H. H., Boas memorial volume, New York, 1906, cited by Robert- 

 son (1907-08).^ 



