Effect of Calf Birth Date and
Milk Supply on Calf Growth
Jim Gerrish, Rick Griebenow
Ron Morrow and Fred Martz
One of the very key questions for cow-calf producers is
timing of the calving season. Many years ago, most Missouri
cows calved in the spring as grass began actively growing.
This system generally matched the cow's peak nutritional
demand with peak forage supply. In this system, lactation
occurred only during the pasture season and not while hay was
being fed. Beginning in the 1950's, winter calving became
more common and was actively promoted by feeder calf
associations and university extension. The primary benefit
was that by having an older calf at weaning, the producer
would sell a heavier calf and increase income. As there are
always other aspects to consider, we had a graduate student
examine long term calving and production records from FSRC to
re-evaluate winter vs spring calving.
Materials and Methods
The cows used in this database include Polled Herefords
with either straightbred, Gelbvieh sired, or Angus sired
calves and Hereford X Gelbvieh cows with Barzona or Angus
sired calves. Performance from a total of 2,288 cow-calf
pairs between the years 1982 and 1994 were used in the
analysis. During the time period of 1987 through 1992, 294 of
these cows were mechanically milked on a regular basis to
determine lactation patterns. Using the lactation data and
corresponding calf weight gain data, we were able to partition
the 2,288 cow-calf pair data into high milking and low milking
cow "herds".
Table 1. Calf data from winter and spring calving cows of
different milk production potential.
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Birth Weaning
Herd Weight ADG Weight
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WL 80.3 a 2.03 a 443 b
WH 85.9 b 2.43 b 525 c
SL 81.5 a 2.03 a 398 a
SH 88.6 c 2.56 c 518 c
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Numbers within a column followed by the same letter are not
significantly different at p<.05.
The calving season at FSRC is February 15 through April
15 so we further divided the herd into approximate winter and
spring groups, with those cows calving prior to March 15 going
into the winter herd and those calving after March 15 going
into the spring herd. We could, therefore, compare 4 herds:
winter-low milk (WL), winter-high milk (WH), spring-low milk
(SL), and spring-high milk (SH).
The statistical procedures used allowed us to remove the
effects of breed, age of cow, and sex of calf from the final
analysis. The results should thus represent a fairly generic
herd situation of mixed breed, average age cows with a 50:50
steer:heifer ratio.
Results and Discussion
Performance of calves from the four cow herds is given in
Table 1. High milk producing calves had significantly heavier
calves at birth than did lower milk producing cows. Even
though calves of high milk producing cows were 5 to 7 lbs
heavier, there was no difference in calving ease between the
two groups. High milk producing cows had a seasonal average
weight of 1124 lb while low milk producing cows averaged 1017
lb. The difference in cow size would have easily facilitated
the larger calves. There was a highly significant, near
linear, relationship between birthweight and weaning weight,
although the correlation was not very great (r2=.16).
As would be expected, calves from high milk producing
cows gained significantly faster than calves from low milk
producing cows. The surprising difference was that spring
born calves from high milk producing cows gained faster than
winter born calves from high milk producing cows. Popular
thinking is that the older winter born calf would be more
likely to gain faster just because it is older and has a
little more size when pasture season comes. Figure 1
illustrates the difference in lactation curves between early
and late calving cows and helps to explain the results.
Spring calving cows came to peak lactation more quickly
after calving than did winter calving cows (63 days vs 74
days). This was a direct impact of the higher nutrient level
supplied from pasture relative to hay. Peak lactation was
also higher for spring calving cows but the duration of peak
lactation was shorter for spring calving cows compared to
winter calving cows. Reduced forage availability occurring
after July 1 probably limited lactation in both groups
resulting in a fairly even decline in milk production after
this date.
The end result of the study was that calves from lower
milk producing cows gained at similar rates for the season and
the earlier born calves were in fact 45 lbs heavier at
weaning. Calves from spring calving cows, on the other hand,
gained at a faster rate and at weaning were equal in weight to
earlier born calves despite being an average of 26 days
younger.
Some critics would say that because the calves are of
different ages, we should make comparison on the basis of
adjusted 205 day weights. But, as the old timers say, you
don't get paid for adjusted weights. Because there is a 26
day difference in age and the statistical analysis has already
factored out effects of cow age and calf sex, the difference
in 205 day weights would be 26 days times 2.56 lb/day or a 67
lb 205 day adjusted weight advantage for the high milk
producing spring calving cows. The low milk producing spring
calving cows would have no advantage on a 205 day weight
basis.
Another parameter that we were able to examine in this
study is the relationship of dam weight to calf weaning
weight. The pendulum of "bigger is better" has already begun
to swing back toward moderation so this bit of information is
just one more nail in the coffin of 1500 lb cows. Figure 2
compares calf weaning weight with their dams weight.
The relationship is highly significant, but as with birth
weight, the correlation is not real tight (r2=.21). Our data
indicated that cows over 1250 lbs quickly become very
inefficient in terms of lbs of calf weaned per lb of cow
maintained. The rate of weaning weight increase begins to
slow significantly around 1100 lb cow weights. These results
suggest that for most cow-calf producers in north Missouri,
1100 lbs is a good target weight for the cow herd.
Practical Implications
The overall results of this study suggest that the
advantages of early calving may be limited in cow herds where
milk production potential is above average. If calves of
comparable size can be produced by calving in late March and
early April compared to late February and early March, the
lower risk weather of early spring may reduce labor
requirements of the cow herd and reduce calf mortality. The
average daily temperature is approximately 12 degrees warmer
in the latter period. The additional nutrient requirements
of lactation during the colder time period increase the cost
of maintaining cows in the winter.
In herds with average or lower milk production potential,
winter calving may still be the most appropriate alternative
if calves are sold at weaning. Retained ownership places a
very different perspective on weaning weights. If calves are
not sold at weaning and are either backgrounded or finished,
weaning weight becomes a fairly unimportant number.
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