A COMPARISON OF DIFFERENT MODELS OF THE LACTATION CURVE IN EGYPTIAN BUFFALOES

Data on 933 records of weekly milk yield representing the first six lactations
of 464 Egyptian buffaloes were utilized in this study. Buffaloes belong to the herd of
the Animal Research Center of the Ministry of Agriculture located in Mahelet Mousa,
Kafr EI-sheikh Governorate. Data represented the period from 1989 to 1999. The
objectives of the study were to describe lactation curve using the linear and the
nonlinear forms of the incomplete gamma function and the inverse polynomial function
and to compare the results obtained. Fitting the linear form of the incomplete gamma
function gave logarithmic values (log a) for the initial milk yield ranged between 3.35
and 3.73 and the values increased with advancing parities from the first to the sixth.
The rate of increase to peak production (b) ranged between 0.205 and 0.293 in the
sixth and the first parities, respectively, however. the rate of decrease after peak
ranged between 0.0192 and 0.0298 in the second and the first parities. respectiV!ly.
All values calculated were significant (P<0.01). Percentages of R2 ranged b e~en
68% and 89%. Regarding the inverse polynomial function. the rate of increase to peak
(Ao) ranged between 0.005 and 0.046 in the sixth and the second parities. respectively
and the values were not significant. The slope of the lactation curve (A,)' ranged
between 0.009 and 0.019 in the fifth and the third parities, respectively. The rates of
decline after peak (A2) were equal for all parities. The values of At and A2 were
significant (P<0.01). except the slope (At) of the first parity. Percentages of R2 ranged
between 95% and 99%. On fitting lactation curve using the nonlinear form. the initial
milk yield (a) ranged between 30.30 and 44.38 in the first and the sixth parities,
respectively. and the values increased with advancing parity. The rate of increase to
peak (b) ranged between 0.169 and 0.270, while the rate of decrease after peak
ranged between 0.017 and 0.029. All parameters were significant (P<0,01) arid
percentages of R2 ranged between 72% and 90%. Time to peak production (weeks).
peak production (kg) and the fitted values of the total milk yield estimated by the three
functions were compared with the corresponding values of the a ct'u a I data. The
nonlinear form of the incomplete gamma function gave better and closer results to the
actual data than the linear form or the inverse polynomial function. Residual'means
squares (RMS) calculated by fitting the three functions were also compared. The
results indicated that fitting the nonlinear form gave rise to the smallest RMS. It is
concluded that the nonlinear form explained the shape of the lactation curve better
than the linear form or the inverse polynomial function.