SELECTIONS - Winter 2003
Pros and Cons of the Half- and Quarter-cc Straws
By Mel DeJarnette, reproduction specialist
While European-based A.I. companies historically have marketed semen
in one-quarter cubic-centimeter (1/4-cc) straws, U.S. A.I. companies have
provided semen in one-half cubic-centimeter (1/2-cc) straws. One U.S. A.I.
marketing organization recently began a transition to 1/4-cc straws, which
has encouraged producers to ask, "Why the difference?" Some of the promotional
materials accompanying this transition even have suggested that the customer
can expect higher conception and fertility with this "new" package size.
So, what's the true story with all this hullabaloo?
A HISTORICAL VIEW
The true story is that the 1/4-cc straw is nothing new. When ampules were
replaced by the French straw back in the late ’60s and early ’70s, the U.S.
A.I. industry, after careful evaluation of each, opted for the 1/2-cc straw,
while A.I. organizations in Europe and later Canada chose to use the smaller
diameter 1/4-cc straw.
FREEZING AND THAWING
Because of its smaller diameter, the quarter-cc straw lends itself to
slightly faster freezing rates and the potential for a slightly improved
post-thaw sperm survival, provided glycerol levels and freezing rates are
adjusted appropriately to accommodate the alternative packaging system.
However, the characteristics of the 1/4-cc straw that allow for this slightly
improved post-thaw survival (large surface-to-volume ratio) also make the
1/4-cc more sensitive to thermal insult during straw retrieval and/or after
thawing.
An extensive, detailed comparison of the thermal sensitivity of 1/4- and 1/2-cc
straws was published in the proceedings of the sixth National Association
of Animal Breeders (NAAB) technical conference in 1976. In that study1,
the effects of thermal insult during on-farm straw retrieval from the
liquid nitrogen storage vessel were mimicked by exposing frozen straws to
room temperature (20 degrees Celsius) for intervals of 15 seconds, 30 seconds,
or one, two and four minutes before plunging back to liquid nitrogen
temperatures. The temperature inside the 1/4-cc straws rose much more rapidly
when exposed to room temperature than did the 1/2-cc straw (Figure 1). And,
the reduction in post-thaw sperm motility in response to these thermal insults
was much more dramatic in 1/4- than in 1/2-cc straws (Figure 2).
The smaller diameter 1/4-cc straw also was found to be more sensitive to heat
and cold shock after thawing. This study suggests inseminators must exercise
a greater degree of both pre- and post-thaw thermal protection during straw
retrieval, gun assembly and A.I. when using 1/4-cc straws to get optimum results.
These facts were major reasons for the U.S. A.I. industry’s decision to use
the 1/2-cc straw. Greater sensitivity of the 1/4-cc straw to thermal insult and
inseminator competence are of little significance in Canada and Europe,
where more than 95 percent of all inseminations are performed by highly trained,
professional technicians whose conception rates can be monitored routinely.
However, in the United States, where most inseminations are performed by the
herd owner or an on-farm inseminator, variation in inseminator skills and
level of training likely will interact with straw type to affect conception rates.
Berndtson and coworkers cited a German study4 that supports this interpretation,
wherein non-return rates of 1/4- and 1/2-cc straws were compared between "good" and
"poor" technicians. While non-return rates of good technicians were similar for
1/4- and 1/2-cc straws, poor technicians achieved higher non-return rates with 1/2-cc
than with 1/4-cc straws (see Table 1). Back in the late 1970s, the variation in
A.I. technician proficiency in the United States and its potential impact on
semen quality and fertility was recognized as a strike against the
smaller-diameter, more thermally sensitive 1/4-cc straws. This is still of
great concern today.
Table 1. Summary of studies comparing the fertility of 1/4- and 1/2-cc straws.
| Reference |
Sub-group |
Straw size |
| Quarter-cc |
Half-cc |
| Percent pregnancies |
| Select Sires, 1972, unpublished |
None |
65 (9,356) |
66 (17,229) |
| Johnson et al., 1995 |
None |
64 (679) |
62 (681) |
| Kroetsch, 1992 |
|
| Study 1 |
20 x 106 sperm/dose
30 x 106 sperm/dose |
69 (2,770)
68 (2,286) |
68 (3,000)
66 (3,035) |
| Study 2 |
None |
66 (5,797) |
66 (6,048) |
| Kupferschmied, 1972 |
"Good" technicians
"Poor" technicians |
71 (2,266)
66 (1,071) |
70 (2,119)
70 (1,071) |
PHYSICAL ADVANTAGES
The 1/2-cc straw is considered more user friendly than the 1/4-cc straw because
it is easier to handle, and it’s easier to read. There also is less breakage
during straw retrieval from the storage tank. Supporters of the 1/4-cc straw point
out that smaller straws require less storage space, which potentially can lower
storage and shipping costs, and reduce the quantity of extender and antibiotics
needed to process a given amount of semen. While these are valid points, all of
these factors combined contribute only a small amount to overall straw
production cost.
PREGNANCY RATES
The argument that 1/4-cc straws result in slight increases in conception and
pregnancy rates is a fallacy, as is the common misperception that the 1/4-cc
straw has less semen.
Regardless of straw type, all reputable A.I. organizations adjust the
semen-extension process and concentration of sperm such that both the
1/2- and 1/4-cc straw will have the same total number of sperm. And the truth
is, that’s what’s really most important-the total number of sperm placed
in the reproductive tract, not the straw size. The reality is that all
straws (U.S., Canadian, European, 1/4- or 1/2-cc) contain two to four times
more sperm than are necessary to get the cow pregnant. Thus, any improvement
in post-thaw survival imparted by the 1/4-cc straw may allow the A.I. center
to achieve the same level of fertility at lower cell numbers per dose and
thereby more efficient utilization semen from short-supply bulls. However,
this should not be expected to translate into higher conception rates in
the field. There are volumes of research to support this interpretation in
addition to the direct comparison studies referenced in Table 1.
SUMMARY
The 1/4-cc straw is absolutely capable of achieving conception rates
comparable to those of the 1/2-cc straw if handled correctly. But the
1/4-cc straw is certainly nothing new, nor is it a short cut to higher
conception rates.
The day may come that Select Sires considers converting to the 1/4-cc straws
to take advantage of some of the benefits they impart. However, it will
not be done under the illusion that our customers will realize higher
conception. Consideration also will be given to customer preferences. In
contrast to other countries, the variance in inseminator competence in
the United States suggests that the potential for reduced conception rates
with the 1/4-cc straw is a distinct possibility that must be considered prior
to such a conversion.
References
- Berndston, W.E., B.W. Pickett and C.D. Rugg. 1976. Procedures for field
handling of bovine semen in plastic straws. Proc. 6th NAAB Tech. Conf. Artif.
Insemin. Reprod. pp 51-60.
- Johnson, M.S., P.L. Senger, C.H. Allen, D.D. Hancock, B.M. Alexander and
R.G. Sasser. 1995. Fertility of bull semen packaged in .25- and .5-milliliter
French straws. J. Anim. Sci. 73:1914-1919.
- Kroetsch, T.G. 1992. Experiences with mini-straws. Proc. 14th NAAB Tech.
Conf. Artif. Insemin. Reprod. pp 64-67.
- Kupferschmied, H. 1972. Untersuchungen uber die umstellung von mittleren
auf feine pailletten in der rinderbeasamung. Zuchthygiene 7:67.
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