Data
Total of 2,239 Hanwoo steer (weight per month) from the Hanwoo Improvement Center, National Agricultural Cooperative Federation in Korea were used to estimate the growth curve. Steers were raised from the 46
th to 57
th progeny test to evaluate candidate breeding bulls. Progeny test was surveyed twice a year. Measurement of body weight for progeny test was started when steers were about 6 months old. The odd number progeny test was started in May, but the even number progeny test was started in November. Each animal in the data was born from 2008 to 2014 and castrated at 5 to 6 months old. Weight was measured from 6, 12, 18 and 24 months old (for each individual, there was some difference in the time of measurement). A total of 8,955 weight data according to age by months were converted to age by day by calculating the difference between measured period and birthday.
Outliers that excessively deviate from the average measured month were removed. In Hanwoo steer, a measured period with a higher growth phase and a lower result than the previous records was removed, because this was considered an error. Finally, after these processes, a total of 8,916 weight-age (day) records were used for growth curve analysis.
Both measured age (day) and weight are summarized in Table 1. As recorded age is increased, it seems likely that the standard deviation of measured weight also increases owing to the influence of environmental factors, but the coefficient of variation does not increase.
Adjustment of body weight data
Before estimating the growth curve of Hanwoo steer, adjustment of weight data was needed, because the range of data from 2,239 steers was too broad from 2008 to 2014. Data were not considered as environmental factors used for estimating the exact growth curve. Measured weight was adjusted by group number of progeny test. Steers with the same group number of progeny test shared similar environment such as the test started in cowshed, test ended in cowshed, tested year, tested season, and birthday. The effect of the same group number of progeny test in body weight was analyzed by general linear model (GLM) procedures using SAS 9.4 package (SAS Institute, Cary, NC) and removed from measured body weight. The statistical model used for adjusting body weight was as follows:
The 57
th progeny tested group was used for reference to estimate the effect of each progeny tested group. The effect of the 57
th progeny tested group was assumed to be zero. The estimated effect of the progeny tested group is shown in Table 2.
Nonlinear model of growth curve and statistical analysis
There were three nonlinear models (Gompertz, Von Bertalanffy, and Logistic) used for estimating the growth curve of Hanwoo steer. All models show estimated weight by age (day) and a sigmoid curve with an inflection point exists. Each model was calculated as follows:
where
W
t = weight at age
t (day)
A = asymptote for weight; mature weight
b = constant of integration
k = intrinsic growth rate
e = natural logarithm.
The inflection point is where the growth curve shape changes from being concave downward to convex upward. In other words, the point where daily gain changes from increase section to decrease section is the inflection point, and daily gain becomes the highest value at that point in the growth model. t is age at inflection point (t
i) when the solution of the twice differentiated growth curve equation becomes
W
ti is weight at inflection point and slope at t
i is daily gain of inflection point.
Weight-age data were fitted to three kinds of nonlinear models using SAS 9.4 (SAS Institute, Cary, NC) and nonlinear regression (NLIN) procedure with Gauss-Newton method.