Growing Pains
Part 5 - How To Get More Heifer Calves
(From SELECTIONS Dairy Newsletter)
By Mel Dejarnette, Reproductive Specialist
I thought that title might get your attention!
Seems like everyone is interested in how to get
more heifers from A.I. these days. Herd expansion
is one reason for a high heifer demand and a second
reason is often less than desirable conception rates.
Because dairy herds typically run a 20 to 30% annual
cull rate, 40 to 60% of the cows must achieve a 12 to
12½ month calving interval simply to maintain herd
size. Higher fertility levels must be achieved if
additional heifers are to be made available for expansion.
Unfortunately, herd expansion and conception rates often
appear to be antagonistic. As herd size increases, herd
fertility tends to decrease resulting in fewer calves born
each year and thus making maintenance of herd size a
difficult task, much less having any extra heifers. The
reasons for this reduced fertility are numerous but are
usually associated with increased stress on cows due to
high production, overcrowding, inadequate nutrition, or
the difficulties associated with heat detection and
proper insemination timing in large herds.
ONLY PROVEN METHOD
Whatever the reason, everyone seems to be interested
in a simple and easy short cut to get more heifer calves.
As far as I know, there is only one time tested and proven
method to get more heifer calves, and that is: get more
calves. For every two calves born, one will usually be a
heifer. In other words, focus on improving conception rates
in your herd and the heifer needs will take care of
themselves.
In the previous four "Growing Pains" articles I discussed
procedures that will facilitate high fertilization rates
in your herd. In general, this involves good heat detection
and proper timing of insemination with high quality,
fertile semen. However, getting the egg fertilized is
just the first step. In order for maximal pregnancy rates
to be achieved, cows and their reproductive systems must
be in sufficient physiological and metabolic states to
support and nurture further development of fragile embryos.
BODY CONDITION SCORING
Body Condition Scoring (BCS) is undoubtedly one of
the greatest indicators available of fertility potential
for a given herd or for a given cow. Body condition scoring
is a visual assessment of the overall fleshiness of the
animal. Body condition is a direct indication of the
amount of fat stored by the animal and thus, of the nutrition
program, in general. The degree of body condition also
reflects the potential energy reserves that will be available
for utilization during periods of suboptimal nutrition
(i.e. post partum negative energy balance).
The following table displays an example of the numerous
controlled research trials, indicating body condition and
the magnitude of loss in body condition after calving. It
shows the tremendous impact BCS has on several important
parameters of reproductive performance. Despite this
overwhelming evidence, routine use of BCS in dairy management
programs appears to be the exception and not the rule.
|
Body Condition Score Loss After Calving |
| |
<.5 |
0.5-1.0 |
>1.0 |
 |
| Days to 1st ovulation |
27 |
31 |
42 |
| Days to 1st estrus |
48 |
41 |
62 |
| Days to 1st service |
68 |
67 |
79 |
| 1st service conception rate % |
65 |
53 |
17 |
|
|
|
(Adapted from Butler and Smith, 1989. J Dairy Sci.) |
The single most critical time for a cow to be in optimum
body condition is at calving. Energy needs during the first
30 to 40 days after calving are so great that it is basically
impossible for cows to consume enough feed to satisfy the
demand. Therefore, all cows will loose weight and body
condition after calving. However, proper feeding and
monitoring of BCS will ensure that weight loss is minimal
and will induce cows to cycle back with fertile heats
sooner after calving. Ideally cows should not loose more
than one BCS postpartum before they start regaining weight.
All cows and heifers should calve with a BCS of about
3.5 ± .25. Therefore, the most important time for you
to score the body condition of your cows is at about
200 days in milk. This allows you adequate time to adjust
rations so that BCS can be gradually adjusted (up or down).
Dramatic changes in BCS during the last month or so of
gestation are often associated with calving problems or
post-partum metabolic disorders.
Use the scoring guide on page 9 to monitor BCS and the
nutritional status of your herd. Remember that BCS is not
only a reflection of your nutrition program, but also of
the fertility potential of your herd. Proper body condition
is essential to ensure that your cows are capable of
maintaining embryonic development once fertilization occurs.
Work with your nutritionist, your veterinarian and your
Select Sires sales representative to establish a regular
routine for body condition scoring your cows and coordinating
needed ration and/or grouping changes.
RECENT RESEARCH RELATED TO GENDER PRESELECTION
Over the years, numerous methods have been suggested to
alter the sex ratio of offspring. One of the most common
is based on the notion that male sperm swim faster than
female, but they die sooner. Thus, if you inseminate early,
the male sperm die resulting in more heifer sperm fertilizing
the eggs. To my knowledge, there is no conclusive, repeatable
data to support the idea that male and female sperm vary in
either swimming speed or longevity.
A recent article published in Animal Reproduction Science
presented some interesting data to support the timing theory.
Using a device which detects changes in vaginal/cervical
mucous around the time of estrus, these researchers varied
the time of insemination and supposedly skewed the sex ratio
of resulting offspring with 90% accuracy. However, the data
set presented was extremely small (13 to 14 animals/treatment).
Until these results are duplicated in other laboratories and/or
on a much larger scale, I’d consider this technology for
altering sex ratio as strictly experimental.
An abstract presented at the 1998 Animal and Dairy Science
meeting from Dr. Ray Nebel’s lab at Virginia Tech, compared
the gender ratios for 822 calvings as affected by time from
first mount to insemination. First mount was identified by
the computerized "Heat Watch" system and time to A.I. and
gender ratios were retrospectively analyzed. Dr. Nebel’s
data (figure 1) would not support the theory that timing
of A.I. has any affect on gender ratios.
Dr. George Siedel and researchers at Colorado State University
also presented some encouraging data at this meeting on their
progress with sexed semen.
Using very low doses of sorted, non-frozen semen and deep horn
insemination in virgin heifers, they achieved approximately
40% pregnancy rates with 90% of resulting offspring being of
the desired sex. However, just as an example of what small
numbers in a data set can do, the frozen control semen in this
project that had not been sorted for sex resulted in 80%
heifer calves.
Sexed semen technology has a way to go before commercial
application will be realized. Among other problems, the
current estimated cost of this technology at about $100 per
straw. Additionally, because the sorting procedure damages
many sperm, it is yet to be determined if acceptable fertility
can be maintained with sperm that are frozen and thawed after
the sorting procedures.
Only time will tell if technology will provide cost effective
methods for altering the sex ratios of A.I. offspring in the
future. At present, however, there is one and only one method
that is available to every dairy and beef producer that has
stood the test of time to get more heifers; get more calves!
BODY CONDITION SCORING
Score
- Individual short ribs have limited flesh
covering, and the ends are sharp to touch.
Vertebrae of the chine, loin and rump
regions are distinct, and hooks and pin
bones are sharp. Severe depressions
between hooks and pin bones are present.
- Individual short ribs are apparent but not
prominent and are sharp to the touch but
with more flesh than 1. Vertebrae of chine,
loin and rump are not highly visible but can
be felt. Area below the tailhead and between
pins is depressed but more fleshed than 1.
- Short ribs can be felt but not easily seen.
Vertebrae of the chine, loin and rump are
rounded and smooth. Areas between the pin
bones and around the tailhead are smooth,
with no signs of fat deposits.
- Short ribs felt only through firm pressure.
The ridge of the backbone in the chine
region is rounded and smooth, and the loin
and rump regions appear flat. Hooks are
rounded, and the area around the tailhead
and pin bones is rounded, with evidence of
fat deposits.
- Bone structure of the backbone, short ribs,
hooks and pin bones is not apparent.
Extreme fat deposits. Tailhead is buried in
fatty tissue.
(Adapted from Wildman et al., 1982)
To order product contact:
Select Sires Inc., 11740 U.S. 42 North, Plain City, Ohio 43064 / Phone: (614) 873-4683 Fax: (614) 873-5751
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