Application of the Wood lactation curve in analysing the variation of daily milk yield in the Zaraibi goats in Egypt

Test-day records (55,161 in total) collected over the period 1995-2008 from 31,762 lactations of 2202 does raised at an experimental station were used to estimate the lactation curve for Egyptian Zaraibi goats. Milk yield was recorded every 2 weeks, including the suckling period. Daily records from does with at least 6 weeks of lactation including low milk yield or short lactations were used. The Wood function was fitted to the milk records with the parameters associated with initial daily yield (a), rate of increase prior to the peak yield (b) and rate of decrease after it (c). The analyses were done only for 1553 lactations from 944 does with positive curve parameters. The Wood function-based estimates (standard deviation) were: peak week (PW) 3.64 weeks (3.28); peak milk yield (PMY) 1.92 kg (0.70); and persistency (S) 1.38 (0.37). Total milk yield (TMY) computed from the daily records had a mean of 253 kg (standard deviation (SD) = 59.9). Genetic variation in all studied traits was analysed using the average information-restricted maximum likelihood (AI-REML) algorithm. The fixed effects considered were parity, year and season of kidding and litter size at kidding. Heritability estimates for the function parameters were low (0.023-0.044). Positive genetic correlation (0.23) was found between PW and PMY while the phenotypic correlation was negative (-0.33). Selection for high initial daily yield may lead to high production, delayed peak, reasonable persistency and high TMY. With the found correlations, selection on PMY would be less effective for increasing TMY than direct selection. Despite the high average number (about 15) of test-day records per lactation, a high proportion of atypical curve parameters was found and the heritability and correlation estimates were low and with relatively high standard errors. The estimate of PW and the related curve parameters had high standard errors. More attention should be paid to the accuracy in measuring milk yield and to starting milk yield recording immediately after kidding to arrive at better estimates for the shape of the lactation curve. (C) 2013 Elsevier B.V. All rights reserved.