Description of Study Area
The study was conducted in the lowland area of Abergelle (Saziba CBBP village) and the midland area of the Sekota (on-station) district in the Waghimra administrative zone of the eastern Amhara region. Abergelle district is located between 12º56´02˝N and 38º57´22˝E. The district receives very little rainfall, with an erratic distribution. The average annual temperature ranges from 23 to 37 degrees Celsius. The Sekota district is located between the longitudes of 12o 43' 651 N and 39o 02'027 E. The minimum and maximum temperatures are 12.2°C and 26.7°C, respectively, with 582.8 mm of mean annual rainfall (Metrology data, 2020). The study areas have rugged topography characterized by mountains, steep escarpments, and deeply incised valleys. The production system is a mixed crop–livestock system with a focus on livestock, mainly goats.
Animal management and data collection
A total of 259 Abergelle goats with a parity of one up to six were used for recording. Physically healthy and free from mastitis disease, goats were used. Udder depth, udder circumference, rear udder attachment, the distance between teats, and teat length were measured using a flexible measuring tape, and udder width and teat diameter were measured with a caliper. Fore and rear udder attachment, medial suspensory ligament, teat placement rearview, teat placement side view, and udder shape were scored subjectively using a nine-point linear scale (Table 1). Data were collected at 14-day intervals beginning after 7 days of kidding. According to ICAR (2018a), milk yield was obtained in the early, mid, and late lactation phases twice in one lactation stage.
Table 1
Linear score udder traits description
Trait | Description |
Score1 | Score5 | Score9 |
Fore udder attachment | Week and loose | Intermediate | Extremely strong and tight |
Rear udder attachment | Week and loose | Intermediate | Extremely strong and tight |
Medial suspensory ligament | Broken ligament | Intermediate | Strong |
Teat placement rear view | Outside | Intermediate | Downwards |
Teat placement side view | In front | Intermediate | At bottom |
Teat formation | Triangular shape | Intermediate | Finger shape |
Udder shape | Faulty | Intermediate | Ideal |
Source ICAR (International Committee for Animal Recording), 2018b.
Data management and analysis
Preliminary data analysis, like the screening of outliers and normality tests, was employed before conducting the main data analysis. A GLM procedure using SAS/STAT® Software, version 9 of the SAS system for Windows (SAS Inst. Inc., 2002), was used for data analysis. Pearson's correlation was used to examine the relationship between milk yield and linear udder traits. The best-fit models were selected based on the coefficient of determination (R2), correlation coefficient (r), and R2 change. The following GLM and Stepwise REG procedure models were used:
Yijk = µ + Mi +Pj +Uk + eijk
Where; Y = observed average daily milk yield
µ = Overall mean
Mi = the effect of ith management (on-station and CBBP)
P j= the effect of jth parity (one to six)
Uk = the effect of kth udder trait (UD cm, UW cm, UC cm, DBT cm, TL cm, RUA cm, FUA, RUA, MSL, TPRV, TPSV, TF, and US)
eijk = random residual error
Y = β0 + β1X1 + β2 X2 + β3X3
Where:
Y = Dependent variable: milk yield
β0 = population constant common to all observations (intercept)
X1-X3 = Independent variables: UC, UD, and UW.
β1-β3 = represent regression coefficients of the independent variables X1 to X3, respectively. Statistically significant differences were declared at P < 0.05.