Dr. John Hough, Chief Science Officer, EPD International, Inc.

Several factors influence a calf's weaning weight. Some are genetic while others
are environmental in nature. Environmental factors include grain or forage type,
amount and quality. Humidity, temperature, rainfall and many other factors
affect a calf's environment. Improvements in management are environmental
factors that can certainly improve calf weaning weights. Direct genetics for
growth also influence a calf's weaning weight. These genetics come from a
calf's parents, one-half from the dam and one-half from the sire. Also, genetics
affect the dam's milk production. To make matters more complicated, a dam's
milk production also is influenced by environmental as well as genetic factors.

These factors can be more easily understood in an equation format.

1
Calf's
Weaning
Weight
= Environmental
Factors +
Sire's
Growth
Genetics
+
Dam's
Growth
Genetics
+
Dam's
Milk
Production
which can be rewritten...
2
Calf's
Weaning
Weight
= Environmental
Factors +
Sire's
Wean Wt
EPD
+
Dam's
Wean Wt
EPD
+
2 times
Dam's
Milk
EPD
An animal's genetic merit is its Breeding Value. An EPD is one-half the Breeding
Value, because a parent only transmits a random one-half of its genes to each
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Understanding Factors that Influence a Calf's Wean
of its progeny. Therefore, from equation 2., the sire transmits a random half of
his genes for weaning growth (Sire's WnWt EPD) to the calf. The dam also
transmits a random half of her genes for weaning growth (Dam's WnWt EPD) to
the calf. The dam provides her entire genetic merit for milk production to the
calf, which is her Milk Breeding Value. The dam's Milk EPD is in terms of
transmitting ability, which is one-half her Milk Breeding Value. In this case we
must consider the Dam's Milk Breeding Value, not her Milk EPD, because the
milk genes transmitted to the calf do not directly affect its own weaning weight,
but rather the actual milk production of the dam. Therefore, the dam's Milk
Breeding Value is used which is equal to twice her Milk EPD.
Examples should help you better understand the factors that influence two
calves' weaning weights. Let's say environmental factors indicate that an
average calf should wean at 500 pounds. This value in a true-life situation is
never known, which is why one can never predict an animal's actual weight.
Calf1's parents are Sire1 and Dam1. The accompanying table gives parents'
EPDs. Calf1's weaning weight = [Environmental Factors] + [Sire1 WnWt EPD] +
[Dam1 WnWt EPD] + [2 times Dam1 Milk EPD] = [500] +[10] + [40] + [2 X 5] =
560 pounds. Calf2's parents are Sire2 and Dam2. Calf2's weaning weight =
[Environmental Factors] + [Sire2 WnWt EPD] + [Dam2 WnWt EPD] + [2 times
Dam2 Milk EPD] = [500] +[10] + [10] + [2 X 20] = 560 pounds. Thus, the
progeny resulting from these two matings would be equal in weaning weight,
even though differences exist in the WnWt and Milk EPDs of the parents.
Considering only the dams, Calf1 gets much of his weaning weight from his
dam's growth genes, while Calf2 get much of his weaning weight from his dam's
milk production.
Parent ID WnWt EPD Milk EPD M&G EPD
Sire1 +10 +25 +30
Dam1 +40 +5 +25
Sire2 +10 -35 -30
Dam2 +10 +20 +25
DamX 0 0 0
Let's take another example. Suppose we now breed Sire1 and Sire2 to DamX.
Sire1's calf weaning weight = [Environmental Factors] + [Sire1 WnWt EPD] +
[DamX WnWt EPD] + [2 times DamX Milk EPD] = [500] +[10] + [0] + [2 X 0] =
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Understanding Factors that Influence a Calf's Wean
510 pounds. Sire2's calf weaning weight = [Environmental Factors] + [Sire2
WnWt EPD] + [DamX WnWt EPD] + [2 times DamX Milk EPD] = [500] +[10] +
[0] + [2 X 0] = 510 pounds. Note, we would expect both calves to weigh the
same at weaning. This is because both sires have the same WnWt EPDs and
are mated to the same dam. The Milk EPD of a sire has absolutely no direct
consequence on his progeny's weaning weights. Only a sire's WnWt EPD
directly influences his progeny's weaning weights. A sire's Milk EPD only has a
direct effect on his daughters' milk production which in turn affects their progeny
(his maternal grand progeny). In this example Sire1 and Sire2 have vastly
different Milk EPDs (+25 versus -35 pounds), but their own calves are expected
to have the same weaning weights if mated to comparable cows. A sire's Milk
EPD does not play a direct role in his own calves' weaning weights, only his
WnWt EPD plays a direct role.
A sire's WnWt EPD is figured in terms of his own progeny. On the other hand, a
sire's Milk EPD is calculated in terms of his daughters' progeny. A sire transmits
a random one-half of his genes to each progeny. The progeny transmit a
random one-half of their genes to their progeny, and so on each subsequent
generation. In other words, 1/2 of an animal's genetics come from each parent,
1/4 of its genetics come from each grand parent, 1/8 of its genetics come from
each great grand parent, etc.
Let's examine expected weaning weights of a sire's daughters' progeny or in
other words his maternal grand progeny. A random one-half of his direct genes
for weaning growth is transmitted to his daughters, this value is his WnWt EPD.
In turn the daughters transmit a random half of their genes to their progeny, or
in other words, one-fourth of the maternal grand sire's direct genes for growth.
This value would be 1/2 of the grand sire's WnWt EPD. Next, remember a sire's
Milk EPD is figured in terms of his daughters' progeny. Therefore a sire's Milk
EPD (not one-half his Milk EPD) is the value that affects his daughters' calves.
Therefore, when we look at a sire's total genetic influence on his maternal grand
progeny, we have a combination of his direct genetic effects for weaning growth
and his maternal genetic effects. This combination is one-half his WnWt EPD
plus his Milk EPD. This is the definition of Milk & Growth EPD (M&G EPD = 1/2
WnWt EPD + Milk EPD). Milk & Growth EPD is the total genetic influence a sire
has on his daughters' calves, both from a direct genetic standpoint as well as a
maternal standpoint.
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Understanding Factors that Influence a Calf's Wean
Whenever a value (M&G EPD) is a combination of two other values (WnWt EPD
and Milk EPD), different combinations of the two values can sum to the same
single value. Let's explain by looking at a sire with a M&G EPD of +20 pounds.
This value could result from WnWt and Milk EPDs of 80 and -20 pounds, or 60
and -10 pounds, or 40 and 0 pounds, or 20 and 10 pounds, or 0 and 20 pounds.
This means heavy calf weaning growth can result from virtually all high genetics
for growth (WnWt EPD) or high maternal genetics (Milk EPD) or any
combination in-between. Average to low Milk EPD parents can have heavy
calves, as can average to low WnWt EPD parents.
This same reasoning is utilized for a dam. She can wean heavy calves because
of her direct genes for weaning growth, because of her genes for milk or any
combination of both. This is why some cows with low Milk EPDs can wean
heavy calves. It's probably because of high WnWt EPDs. The same reason also
is why some cows with low WnWt EPDs can wean heavy calves. Likewise, it's
probably because of high Milk EPDs.
Remember, when you look at a calf's weaning weight, several factors play a
role. The environment has a very large effect as does the combination of growth
and milk genes. Never try to predict a cow's production by looking at her WnWt
EPD alone or her Milk EPD alone. It's always a combination of both. Also
remember, a sire's Milk EPD has no direct effect on his own progeny. His Milk
EPD only influences his daughters' calves, only his WnWt EPD affects weaning
weights his own progeny.