Use of Wood's model to analyze the effects of milking methods on lactation curve in sheep

One hundred and sixty-six lactating Makui breed sheeps (2-6 years old) were used to investigate the effects of two milking methods (milking with oxytocin (OT) and milking without oxytocin (HT)), ewes age and stage of lactation on milk yield and composition during 168 days of lactation and to estimate shape of lactation curve using Wood's model. Oxytocin treatment (OT) had a significant effect on the average daily milk yield (ADMY) and total milk yield (TMY) (P < 0.0001). TMY for OT ewes was 45.61% (P < 0.01) greater than that in HT ewes. OT ewes had significantly (P < 0.01) longer lactation period (LL) than HT ewes. The effects of ewes age on ADMY, TMY and LL were significant (P < 0.01). In both milking methods, 4-, 5- and 6-year-old ewes produced significantly (P < 0.01) greater ADMY and TMY than 2 years old ones. Stages of lactation had a highly significant (P < 0.0001) effect on milk yield. Except (b) parameter, (a) and (c) parameters of Wood's model were significantly (P < 0.05) affected by milk measurement methods. In addition, effects of ewes age on the model's parameters were significant (P < 0.05). TMY obtained by Wood's model was not affected (P > 0.05) whereas peak time (PT), peak yield (PY) and persistency (P) were significant (P < 0.05) as compared with Fleischmann method for two milking methods. OT ewes reached PT (day) earlier than HT ewes. OT method had a significant effect on fat and SNF percent (P < 0.05) whereas percent of protein and lactose unaffected by OT and HT methods (P > 0.05). In both methods, stages of lactation had a highly significant effect on milk fat, protein and SNF percentage (P < 0.05). The results of this experiment showed that oxytocin has galactopoietic effect due to the contraction of myoepithelial cells maybe limiting the effects of milk-borne negative feed-back mechanism on milk secretion and Wood's model overestimated TMY and PY whereas underpredicted PT and P.