Semen quality in bulls
Understanding the challenge of successful freezing
By Ashley R. Hartman, David M. Grieger and Jason M. Warner
Breeding soundness exams (BSE) have been developed to evaluate the ability of a bull to successfully service females. While a BSE is an assessment at a single point in time, it is common practice for bulls to undergo an exam both prior to first use and annually as herd sires. This is warranted as the industry average BSE failure rate has been estimated to be as high as 20% in yearling bulls. However, if a bull collected for A.I. passes a BSE it does not necessarily mean his semen will meet qualifications for freezing. Reasons for this are not well understood nor are there realistic expectations for bulls based on age.
To better understand these challenges, and the implications they have for both seedstock producers and commercial collection facilities, research was recently conducted at Kansas State University utilizing data from the Kansas Artificial Breeding Services Unit, Manhattan, Kansas (Hartman et al., 2022; Challenges Associated with Semen Quality While Collecting Beef Bulls for Semen Freezing (newprairiepress.org))[1].
Data were analyzed on a total of 14,750 ejaculates from 906 bulls collected from January 2008 to December 2018. Bull birthdates were provided, and bull age was calculated in months from birthdates supplied to the collection date of each ejaculate. Bulls were assigned to one of four age groups: ≤ 12 months, 13 - 18 months, 19 - 30 months and > 60 months. Bulls were collected twice weekly on Mondays and Thursdays using an artificial vagina as the preferred collection method. Bulls not receptive to the mount steers or the artificial vagina after three or four attempts were subjected to electro-ejaculation for ejaculate collection.
Ejaculates were required to meet quality standards which included pre-freeze progressive motility of equal to or greater than 50%. The ejaculates could not contain greater than 30% abnormal spermatozoa and must have had a progressive motility of at least 30% post-thaw to pass freezing quality standards. Although these samples were collected to freeze semen, secondary quality standards were set based on bull BSE requirements as determined by the Society of Theriogenology. To be considered a satisfactory breeder the threshold for progressive motility was 30% or greater, and sperm abnormalities of less than 30% total morphologically abnormal sperm with less than 20% of those abnormalities being head defects.
Over the 10 years, only 21% of all ejaculates collected met the freezing quality standards of 50% progressive motility pre-freeze, 30% progressive motility post-freezing, and the sperm morphology requirements of less than 30% abnormal spermatozoa with less than 20% of those being from primary sperm abnormalities (Figure 1).
When evaluating ejaculates for progressive motility, 11% of all ejaculates collected were not high enough (≥ 30%) to be considered a satisfactory breeder, and 63% of all ejaculates did not reach the initial progressive motility threshold for freezing (≥ 50%). Of ejaculates collected from bulls ≤ 12 months of age 15% did not meet minimum progressive motility requirements to pass a BSE (30%) (Figure 2).
Six percent of ejaculates from bulls 13-18 months of age were unsatisfactory for progressive motility for a BSE, and ejaculates from bulls 19-30 months of age did not meet the standard 10% of the time. Ejaculates collected from bulls ≥ 31 months of age failed to meet BSE progressive motility standards 14% of the time. When evaluating the likeliness to freeze semen, the percentage of ejaculates meeting quality standards was much lower.
Ejaculates collected from bulls ≤ 12 months of age failed to meet freezing quality progressive motility 64% of the time, while ejaculates from bulls 13-18 months old failed 58% of the time and ejaculates from 19–30-month-old bulls failed 61% of the time. Ejaculates from bulls ≥ 31 months of age did not meet the progressive motility threshold 67% of the time.
These results suggest that when collecting bulls ≤ 12 months and ≥ 31 months of age, progressive motility may pose a challenge in collecting semen that meets the standards to be frozen.
For bulls whose ejaculates were of freezing quality for both pre-freeze and post-freeze progressive motility, those samples were evaluated for percentage of abnormal spermatozoa. When evaluating ejaculates for primary sperm abnormalities, 87% had less than 20% primary sperm abnormalities, which would be considered satisfactory for a BSE and for freezing semen (Figure 3). Unlike the results for motility as bulls aged, the percentage of primary abnormalities in the ejaculates decreased with advancing age, suggesting an increase in ejaculate quality. Ejaculates from bulls ≤ 12 months of age had the greatest number of primary abnormalities with 24%, while ejaculates from bulls ≥ 31 months of age had an average of 10% primary abnormalities.
When evaluating total sperm abnormalities per ejaculate, 77% of ejaculates met the threshold of less than 30% total abnormalities for both BSE and freezing quality (Figure 3). Ejaculates from bulls ≤ 12 months of age failed to meet the total sperm abnormality maximum (30%), 28% of the time. Ejaculates from bulls 13-18 and 19-30 months of age exceeded the total abnormality threshold 25% of the time, and ejaculates from bulls ≥ 31 months of age exceeded the maximum total abnormal spermatozoa 20% of the time. These data indicate that collecting semen that passes freezing quality standards is a clear challenge in bulls, independent of a sire’s ability to pass a BSE, and may be related to age.
References[1] Hartman, A. R., N. M. Goodenow, S. K. Tucker, K. E. Fike, and D. M. Grieger. 2022. Challenges associated with semen quality while collecting beef bulls for semen freezing. Kansas Agricultural Experiment Station Research Reports: Vol. 8: Iss. 1.
Hartman is a graduate teaching assistant, Grieger is a professor in reproductive physiology and Warner is an Extension cow-calf specialist, all with Kansas State University.
