Estimation of Lactation Curve parameters
The average daily milk yield was estimated as 12.57±0.01 kg for 281698 daily milk yield records of 750 crossbred cows. The average lactation curve parameters of crossbred cattle were estimated under various non-linear models (Table 2). The parameter ‘a’ was found to be positive and varied between 0.285 kg for the Inverse Polynomial Model to 16.336 kg for the EDF. Followed to EDF, PEM estimated high and positive ‘a’ parameter whereas the ‘a’ parameter values for GTF, MLF, and PRM were found to be 10.180±0.279, 7.213±0.292 and 5.106±0.276, correspondingly. Also, the parameter ‘b’ showed variation across the different non-linear models. The estimates of the parameter ‘b’ ranged from -1.444±0.025 (MLF) to 0.141±0.008 (GTF). Positive parameter ‘b’ was found in IPM and GTF with corresponding values of 0.0484±0.0003 and 0.141±0.008 while negative parameter ‘b’ was observed in the PEM, MLF and PRM which had values as 0.0004±0.0002, -1.444±0.025 and -0.097±0.0023, respectively. Similarly, differences were seen among the various non liner models fitted to the 305 days milk yield for the ‘c’ parameter values. In models having three parameters, ‘c’ parameter ranged from 4.84×10-6 ±4.81×10-7 (PEM) to 4.668±0.120 (MLF). In the EDF, IPM, GTF and PRM, the ‘c’ parameter was positive and their corresponding values were 0.0018±4.3×10-5, 0.000194 ± 1.82×10-6, 0.0031±7.4×10-5 and 0.00013±4.742×10-6 respectively. Parameters ‘d’ and ‘f’ in PRM were 4.240±0.099 and -6.523±0.1516, respectively and, showed increasing slope of lactation curve. For Gamma type function, the daily milk yield raised from calving to peak production of 15.145 kg reached on day 45th and then decreased gradually to dryness. The modelling of lactation curve provides guidelines in formulating farm managerial practices in dairy cows. The average initial daily milk yields (kg) estimated under all nonlinear models except EDF were low as compared to previous findings reported by
Rekik et al., (2006), and
Chegini et al., (2015). However average initial daily milk yields of all nonlinear models excluding IPM were higher than that found by
Subham et al., (2017) for crossbred cattle. The average ‘a’ value estimated using EDF was also higher than that of Cole and Null (2009) who reported for dairy cattle under Wood model (‘a’=13.01 kg). Occurrence of variations in the parameter ‘a’ value may be due to differences in genetic groups or in herd management.
The average b and c values in the IPM and GTF were in the scale of the previous studies as reported by
Gradiz et al., (2009) and
Chegini et al., (2015). The b parameter values in all nonlinear models were also lower than previous studies reported by
Suham et al., (2017), Yogesh et al., (2017) for crossbred cattle. The c parameter values estimated under MLF was higher than reported by
Suham et al., (2017) and
Yogesh et al., (2017) for crossbred cattle. The production at the peak estimated under GTF was low compared to the study by
Chegini et al., (2015) and
Khalifa et al., (2017) and reported high as compared to the findings of
Gradiz et al., (2009) and
Cankaya et al., (2011). The persistency of lactation found in GTF was also higher than that found for Holstein cattle
(Tekerli et al., 2000; Rekik et al., 2006; Atashi et al., 2007). Existing differences in these parameters might be the result of a combination of genetic, diet, management and specific climatic effects. The positive parameter ‘a’ in all models clearly indicated that this parameter explained the increasing part of the lactation curve. Based on the sign/direction of the parameters ‘b’ and ‘c’ obtained in the present study, it was determined that IPM curve followed by GTF which applied to fit the 305 days milk data were typical standard curve for crossbred dairy cattle.
Relationship between lactation curve parameters
The Estimates of phenotypic correlation among lactation curve parameters in crossbred cattle was presented in Table 3. The results of the present study clearly indicated that the relationships between parameters b and c for IPM, MLF and PRM were highly significant (P<0.01), negative and high with corresponding values of -0.890, -0.973 and -0.977, in that order while the correlation between b and c parameters in the PEM (0.965) and GTF (0.918) models were positive, highly significant (P<0.01) and higher. For the PRM, the values of correlation between b and f and c and d parameters were negative (P<0.01). Parameters a and b were associated negatively and significantly (P<0.01) for PEM (-0.874), IPM (-0.811) and GTF (-0.981) whereas correlation between a and b parameters of PRM and MLF were highly significant (P<0.01), positive and high with respective values of 0.886 and 0.885, respectively. The parameters a and c were associated negatively (P<0.01) for PEM (-0.755), MLF (-0.967), GTF (-0.833), and PRM (-0.757) whereas correlation between a and c parameters of EDF, and IPM were positive (P<0.01). The relationships among all the lactation curve parameters are important, specially between b and c parameters because (a) is always positive and influence the average level of production (Ali and Schaeffer 1987). The negative relationship between b and c observed in the IPM, MLF and PRM models clearly demonstrated that high daily milk production may be maintained throughout the lactation which in turn has high implication for economic return of the dairy producers; but the positive relationships between b and c found for PEM and GTF low persistency hence high milk production may not be maintained. The negative correlation between a and b parameters found in the present study for the PEM, IPM and GTF clearly showed that the crossbred cows with smaller initial daily milk production might have high peak milk yield. However, the positive association between parameters a and b observed in our study for the PRM and MLF noticeably indicated that the crossbred cows with high initial daily milk production would have low peak milk production; but average milk yield over the complete lactation could be high in both cases.
Fitting of lactation curves with daily milk yield
Lactation curves of observed versus predicted 305 days daily milk yield (Kg) have been presented separately (Fig 1-6) to illustrate the fitness of all the models used in the study for crossbred dairy cattle. The Polynomial regression model gave highest fit to the daily milk yield data (Fig 6; Table 4) with R2, MSE, AIC and CV values of 98.10%, 0.087, -743.31 and 2.37 %, respectively. The Inverse Polynomial Model have also best fitted the observed daily milk yield data with predicted daily milk yield data (Fig 1) with highest R2 (98.05%), lower MSE (0.089), low AIC (-735.8972) and lower CV (2.40%) values. The fitting of observed daily milk yield data with predicted ones were also found to be higher in the Mixed Log Function (Fig 3), and Gamma-Type Function (Fig 2). The R2, MSE, AIC and CV estimates observed for Mixed Log Function and Gamma-Type Function were 96.46%, 0.159, -558.16 and 3.21%; 95.85%,0.190,-505.24 and 3.50%, respectively. However, the Exponential Decline Function and Parabolic Exponential Model depicted relatively low fit to the daily milk yield data when compared with the other non-linear models used in the study. The R2, MSE, AIC and CV values for Exponential Decline Function were 86.02%, 0.54, -183.69, 5.94%, respectively whereas the Parabolic Exponential Model described the milk yield data of crossbred dairy cattle with R2, MSE, AIC and CV estimates of 89.61%, 5.29, -253.67 and 0.43%, respectively. Coefficients of determination (R2) values have been used to evaluate the fit of the models in some studies
(Akbaş et al., 2006). The models that gave the highest R2 values have been accepted as the best fitting models. Based on the values of model parameters investigated in the present study, nonlinear models namely EDF and PEM could adequately fit the daily milk yield data for 305 days lactation; while PRM, and IPM models followed by MLF and GTF models gave best fit and reliable description to the lactation curve pattern and characteristics of crossbred cattle. However only two models namely IPM and GTF could be chosen with corresponding developed equations for predicting daily milk production from calving to 305 days in milk of crossbred dairy cattle managed under DLF of GADVASU because IPM and GTF curves were standard typical curves for the cattle.
Singh et al., (1998) reported that Inverse Quadratic Polynomial (IQP) model was the best function in explaining the first lactation curve based on monthly as well as weekly milk records of Jersey x Sahiwal F1 cows whereas
Yogesh et al., (2017) for Gir crossbreds with R2=90.50% and Cole and Null (2009) for dairy cattle with R2=91.25% under Gamma Type and
Tekerli et al., (2000) reported that Log Transformed Gamma Function gave best fit to daily milk yield of Holstein cows with R2 = 70.80%.