The application of a mechanistic model to analyze the factors that affect the lactation curve parameters of dairy sheep in Mexico

Pollott́s mechanistic model has been designed to describe lactation curve parameters based on the known biology of milk production and can be useful for analyzing the factors that affect this process. A total of 553 lactations (9956 weekly test-day records) of crossbred dairy sheep from four commercial farms located in Mexico, were analyzed to investigate environmental factors that influenced lactation curve parameters, using Pollott’s 5-parameter additive model. This model was fitted to each lactation using an iterative nonlinear procedure. The estimated parameters were maximum milk secretion potential (MSmax), relative rate of increase in cell differentiation (GR), maximum secretion loss (MSLmax), relative rate of decline in cell numbers (DR) and the proportion of parenchyma cells dead at parturition. A general linear model procedure was used to determine the effect of type of lambing, lambing number, flock and lambing season on total lactation milk yield (TMY), lactation length and estimated parameters of the Pollott model. Ewes had an average milk yield of 74.4 L with an average lactation length of 140 days. Flock had a significant (P < 0.05) effect on most of the analyzed traits, which can be explained by the different farmś management practices. The TMY were significantly (P = 0.005) higher for twin-lambing than single-lambing lactations. Sheep in their first lambing had lower TMY than those in their fourth lambing (P = 0.01), possibly explained by the lower values of MSmax (2.85 vs. 5.3 L) and the decrease in DR throughout life (P = 0.03). However, the relative GR was greatest (P = 0.04) during first lambing and then decreased as lambing number increased. Both lambing number and type of lambing also affected milk yield. The parameters of the Pollott model can be useful to explain, with a biological approximation, the dynamics of differentiation, secretion and death of mammary cells in dairy sheep

Comparison of Mathematical Models Applied to F1 Dairy Sheep Lactations in Organic Farm and Environmental Factors Affecting Lactation Curve Parameter

The objective of this study was to compare the goodness of fit of four lactation curve models: Wood’s Gamma model (WD), Wilmink (WL), and Pollott’s multiplicative two (POL2) and three parameters (POL3) and to determine the environmental factors affecting the complete lactation curve of F1 dairy sheep under organic management. A total of 5,382 weekly milk yields records from 150 ewes, under organic management were used. Residual mean square (RMS), determination coefficients (R2), and correlation (r) analysis were used as an indicator of goodness of fit for each model. WL model best fitted the lactation curves as indicated by the lower RMS values (0.019), followed by WD (0.023), POL2 (0.025) and POL3 (0.029). The four models provided total milk yield (TMY) estimations that were highly correlated (0.93 to 0.97) with observed TMY (89.9 kg). The four models under estimated peak yield (PY), whereas POL2 and POL3 gave nearer peak time lactation estimations. Ewes lambing in autumn had higher TMY and showed a typical curve shape. Higher TMY were recorded in second and third lambing. Season of lambing, number of lambing and type of lambing had a great influenced over TMY shaping the complete lactation curve of F1 dairy sheep. In general terms WL model showed the best fit to the F1 dairy sheep lactation curve under organic management