A study on the lactation curve characteristics of grade and Iranian purebred Holstein cows with the use of raw, fat-corrected, and energy-corrected milk test day records

Introduction: In dairy cattle enterprise, there are several economic traits associated with productive and reproductive performance which milk yield is the main source of income for the herd keeper. Knowledge of lactation is considered a useful tool for management decision-making. Based on this, lactation curve modeling can be utilized as an appropriate guide for planning nutrition programs as well as genetic selection schemes. So far, main research has been focused on using raw (unadjusted) milk test day records to model the shape of the lactation curve by applying a variety of linear and non-linear mathematical models and a few research have been carried out to use adjusted milk test day records in terms of fat and protein percentages. The main objective of the present research was to compare lactation curve parameters of Iranian first-parity dairy cows among three types of milk test day records including raw milk (RM), fat-corrected milk (FCM), and energy-corrected milk (ECM).
Materials and methods: The initial data set was provided by the Animal Breeding Center and Promotion of Animal Products of Iran. Editing of the data was carried out by FOXPRO and Excel software based on some criteria such as age at first calving (was set to be in the range of 18-48 months), minimum (3 kg), and maximum (99.9 kg) of RM. There were a minimum of five test day records for each cow during the lactation period. Final data consisted of a total number of 1,902,071 milk test day records belonging to 226,255 first-parity cows (progeny of 5,094 sires and 178,390 dams) distributed in 797 herds and calved during 1996-2015 were utilized. For a mathematical description of the shape of the lactation curve, Wood’s incomplete gamma function was used. The function has three parameters including parameter a (associated with initial milk yield), parameter b (associated with the inclining slope of the lactation), and parameter c (associated with the declining slope of the lactation). Wood's function was fitted to the records of RM, FCM, and ECM of individual cows using SAS software. Based on the estimated parameters, lactation characteristics including peak time (PT), peak yield (PY), and persistency (Per) were subsequently calculated for each cow. Estimated parameters and calculated lactation characteristics were then subjected to a fixed linear model in which the effects of herd, year of calving, the season of calving, age at first calving, genotype group (grade or purebred Holstein), type of milk test day record (RM, FCM or ECM) along with some two-way interactions were included.  
Results and discussion: The results showed that FCM had the greatest parameter a as compared with RM and ECM (P<0.0001). The greatest magnitude of the parameters b and c were found for RM (P<0.0001). Later PT and higher Per were observed for ECM (P<0.0001), while the highest PY was detected for RM (P<0.0001). For all types of records, minimum and maximum PY were observed for the cows calving in the spring and autumn seasons, respectively. Later PT was observed for ECM in all calving seasons except autumn. As compared to RM and FCM, cows calved in spring and summer had the highest persistency based on ECM while cows calved in autumn and winter were found to have more persistence based upon RM compared with FCM and ECM. The least-square means of peak time (PT) in grade cows were found to be 77.74, 69.5, and 79.85 d for RM, FCM, and ECM, respectively, while the corresponding figures for purebred Holstein cows were 80.27, 72.89, and 83.15 d, respectively. In grade cows, least-square means of peak time (PY) were found to be 32.02, 29.23, and 30.70 kg for RM, FCM, and ECM, respectively, while the corresponding figures for purebred Holstein cows were 32.51, 29.56, and 31.14 kg, respectively. For both purebred Holstein and grade cows, minimum and maximum persistency (Per) were obtained for FCM and ECM, respectively, which were significantly different from each other (P<0.001). For RM, FCM, and ECM, annual change trends of PT were found to be 2.132, 2.306, and 2.293 d, respectively, while the corresponding figures for PY were 0.408, 0.369, and 0.395 kg, respectively, and for Per were 0.037, 0.036, and 0.035, respectively. All trends were statistically significant (P<0.0001).
Conclusions: The finding of the present research revealed that purebred Holsteins reach the peak time later than grade cows which is an appropriate characteristic of the lactation curve. Based upon energy-corrected milk yield, peak time as well as persistency was found to be greater than those for raw milk and fat-corrected milk yields. Therefore, the use of energy-corrected milk yield could be suggested to be applied as different experimental nutritional treatments are to be compared in terms of the persistency of the cows. All lactation characteristics of Iranian dairy cows were found to be changed favorably over time.