Modeling the lactation curve for test-day milk yield in Murciano-Granadina goats

Test-day yield measured at intervals of 4 weeks was used to investigate the mathematical description of early stages of lactation and entire lactation curves in dairy goats under field conditions. Characteristics of the lactation curve (maximum yield and time of maximum yield) were calculated separately by parity groups (first, second and third or greater). Test-day data from 733 Murciano-Granadina goats were fitted with five published empirical standard lactation curve models including variations of the incomplete gamma function of Wood, a linear decline and a diphasic model. The average milk yield was predicted with a high degree of accuracy (R-2 > 0.98) by all models for all parities. Peak yield took place at about 4.4 weeks of lactation for first parity and 3.2 weeks for second parity and predicted peak yield ranged from 1.6 to 1.74 kg per day for first parity and from 2.34 to 2.57 kg per day for second parity. For parity 3 or greater, a decline phase with no peak was observed and predicted peak yield at the second week of lactation ranged from 2.66 to 2.79 kg per day. The residual showed low fluctuation around zero for the diphasic model and lower mean absolute deviation, indicating a more satisfactory description of the lactation curve. Of the five models, the diphasic model resulted in the best goodness of fit and gave a good description of the Murciano-Granadina lactation curve for commercial dairy herds when test-day yield from Official Milk Control is monitored. (C) 2002 Elsevier Science B.V. All rights reserved.