Yearling scrotal circumference prediction equation and age Yearling scrotal circumference prediction equation and age adjustment factors for various breeds of beef bulls adjustment factors for various breeds of beef bulls

Scrotal circumference measurements and other data were collected on 4,218 Angus, Red Angus, Brangus, Charolais , Gelbvieh, Hereford, Polled Hereford, Limousine, Salers, and Simmental bulls born in the spring of 1991. All were participants in selected on-farm and central bull tests. Our objectives for the study were to develop 205-and 365-day age-adjust-ment factors for scrotal circumference and derive a 365-day scrotal circumference prediction equation based on adjusted 205-day scrotal circumference. We determined that a 205-day scrotal circumfer ence of approximately 21 cm is necessary to reach 32 cm at 1 year. Age-of-da m adjustment factor for 205-day scrotal circumference is +.8 cm for 2-and 3-year-old dams. The factor for 365-day circumference is +.6 cm for 2-year-old dams.


Introduction
Previou s research has indicated an important relationship between yearling scrotal circumferenc e of beef bulls and the semen traits: sperm motility, percent normal sperm, percent primary abnormalities, percent secondary abnormalities, semen volume, sperm concentration, and total s perm.Yearling scrotal circumferenc e also has been found useful in describing age at puberty.The Society of Theriogenology re c ommends a minimum scrotal circumference of 30 cm for yearling bulls to ensure satisfa c tory reproductive performance.Man y cattleman prefer bulls with yearlin g scrotal circumferences at least 32 cm.The limited information available indicates a high relatio nship between weaning and yearling scrotal circumference.
Yearling scrotal circumference has been reported as moderately to highly heritable.Yearlin g scrotal circumference in bulls also is correlate d highly with age at puberty and with some performance traits of their female offspring.
Our objectives were to develop 205-and 365-day scrotal circumference age adjustment factors and derive a 365-day scrotal circumference prediction equation based on adjusted 205-day scrotal circumference.

Experimental Procedures
Scrotal circ umferences and other data were collecte d on 4,218 bulls born in the spring of 1991.The breeds included Angus, Red Angus, Brangus, C harolais, Gelbvieh, Hereford, Polled Hereford, Limousin, Salers, an d Simmental.All were participants in selected on-farm and central bull tests starting in the fall of 1991.Three scrotal circumference measurements were taken; at the start of the test, midway throu gh, and at the end of the test.At each measurement , weight and date also were recorded.The measuring proced u re is described in the Manual for Breeding Soundness Examinatio n of Bulls (Journal of Theriogenology) .Other information collected included location of test, pedigree information, and age of dam.
The 205-day scrotal circumference adjustment factors were developed u sing least squares analysis by breed, while limiting the age range to 160 to 250 days.The model included contemporary group as a fixed effect and age as a regression.The adjustment factor the linear regression of a ge on scrotal circumference.The procedure for the 365-day scrota l circumference adjustmen t factor was identical, except the age range was 320 to 410 days.A 365-day scrotal circumferenc e prediction equation, based on adjusted 205-day scrotal circumference, was develope d by regressing adjusted 365-day scrotal circumference on adjusted 205-day scrotal circumference for each breed.
Least squares procedures were used to determine an age-of-dam effect for both 205and 365-day scrotal circumferences.The age range again was limited from 160 to 250 days for the 205-day measurement and 320 to 410 days for 365 days.The model included contemporary group, breed, age of dam, and age of calf.Ages of dams were gro u ped into five categories: 2, 3, 4, 5-8, and 9+ years.

Results and Discussion
Table 1 shows the adjustment factors for each breed that w ould enable breeders to adjust scrotal circumferences to 205 or 365 days of age.As indicated by the differences between weaning and yearling factors, scrotal circumferenc e does not increase at the same rate between w eaning and yearling ages.These 365-da y scrotal circumference adjustments were slightly higher than some studies have indicated.
Other studies have found age of dam to have a significant effect on scrotal circumference.These adjustment factors should not be used within ou r prediction equations, but rather as a means of comparing individual bulls for selectio n purposes.
For 205-day scrotal circumference , bull calves out of 2-and 3-yrold dams should be adjusted by adding .8cm.For 365-day scrotal circumference, calves out of 2-yr-ol d dams should be adjusted by adding .6 cm.
The "B" values from Table 2 were used to estimate adjusted 205-day scro t al circumference necessar y to average 32 cm at 1 year.With the exception of Herefords, the breeds were fairly similar.In general, bulls needed about 21 cm scrotal circumference at weaning to reach 32 cm at 1 year.
Age adjustment factors for 205-and 365day scrotal circumferences will allow more accurat e comparisons between bulls.Adjusted scrotal circumferences will make selection more accurate, ju st as adjusted 205-day weights have made selection for weaning weight more accurate.
Becaus e many bull buyers prefer yearling scrotal circumferences of at least 32 cm, seedstoc k producers could reduce costs by eliminating those bulls at weaning that would likely fail to reac h 32 cm at a year.The 365-day scrotal circumference prediction equation and the table of minimum scrotal circumferences should serve as guidelines for producers to identify and cull those individuals.The age-ofdam adjustments can make comparisons involving calves out of younger dams more accurate.
As an example, assum e that an Angus bull from a 2-year-old d am is 220 days old when his scrotal circumference was measured as 22 cm.His adjusted 205-day scrotal circumference would be [22 + .0856× (205 -220) + .8]= 21.5 cm .The adjustment factor of .0856came from Table 1, and the age-of-dam adjustment would be .8cm.The pred i cted 365-day scrotal circumferenc e would be [1.54 × 20.7] = 31.9cm.The regression coefficient (B) of 1.54 was taken from Table 2.When predicting the yearling scrotal circumferen c e, the 205-day ageof-dam adjustment would not be used.
Anothe r example migh t be two Simmental bulls in a yearling contemporary group.One bull had a 36 cm scrotal circumference measured at 352 days of age, and his dam was 2 years old.His adjusted 365-day scrotal circumference would be [36 + .0543× (365 -352)] = 36.7 cm, plus .6 cm for age-of-dam adjustment = 37.3 cm.The second bull was from a m ature dam and had a measured scrotal circumferenc e of 35.5 cm at 370 days of age.His adjusted 365-day scrotal circumference would be [35.5 + .0543 × (365 -370)] = 35.2cm, or 2.1 cm less than that of the first bull.

Table 2 . Regression Coefficients (B) to Predict Yearling Scrotal Circumference and the Weaning Scrotal Circumference Needed to Expect a Yearling Scrotal Circumference of 32 cm
The adjusted 205-day scrotal circumference multiplied by B gives the b expected 365-day scrotal circumference.The 205-day scrotal circumference needed to produc e an average yearling scrotal circumference a Regression coefficients.c of 32 cm.