D
Dr. Carel Muller
Western-Cape Department of Agriculture
Directorate: Animal Sciences, Elsenburg
airy herds generally operate on low profit
margins. This is usually because dairy farmers have little control over the price of milk
as well as input costs. Increasing milk prices
generate a positive feeling in the industry although this
is usually tempered by increasing feed or input prices.
As dairy farming is difficult requiring large time and
personal inputs from workers and managers, indicators
that show a loss in farm income are often missed or
misinterpreted. Indicators that show farm losses even
though the dairy operation is ongoing, are the following: a late age at first calving of replacement heifers,
i.e. past 24 months of age, a high number of first lactation cows in the herd (indicating a high replacement
rate), a high average number of days-in-milk (DIM) of
lactating cows and a low average lactation number for
all the cows in the herd. For instance: by increasing age
at first calving by one month, the cost of raising heifers
is increased by R500 to R1000 per heifer without a significant improvement in milk yield in the subsequent
lactation. A high average number of DIM indicates that
a large proportion of cows in the herd are in late lactation when milk yield is low. A high replacement rate as
indicated by a large proportion of first lactation cows
in the herd also affects farm income as this indicates
the number of cows being lost from the herd. A herd
with a larger proportion of older cows produces more
milk per day than a so-called young herd. In this article
the effect of replacement rate on the internal herd
growth is discussed.
Internal herd growth
Internal herd growth (IHG) simply indicates the growth
in the number of cows in a herd. Usually this should be
estimated at a fixed point each year, i.e. coinciding
with the tax year for instance. For estimating the IHG,
four numbers are required, i.e. the number of cows in
the herd at the beginning (A) and end (B) of the year,
the number of cows sold for dairy purposes (C) and the
number of cows purchased during the year (D). The
number of cows at the beginning of the year is subtracted from the number of cows at the end of the
year after which the number of cows sold for dairy purposes is added followed by subtracting the number of
cows purchased during the year. The following equation describes the IHG or increase in herd size (E):
E = (B - A) + (C - D)
Dividing the increase in herd size (E) by the number of
cows at the beginning of the year (A) reflects the internal herd growth rate (%). A negative IHG value indicates that the herd is losing size requiring purchasing
cows to maintain herd size. A positive IHG rate of 1 to
5% reflects a slow growing herd providing some flexibility while a growth rate of 5 to 15% indicates a fast
growing herd. This provides an opportunity for herd
expansion or for animals to be sold for dairy purposes
creating an additional farm income. A heavier voluntary culling rate also becomes possible especially
among first lactation cows. Factors affecting the IHG
include the number of cows calving down per year, cull
rate of cows, heifer to bull ratio and survival of heifers
from calving to first calving and age at first calving of
replacement heifers.
Herd cull rates
Many dairy herds suffer from high cull rates resulting
in a limited number or no cows available for sale for
dairy purposes. In the following figure the effect of cull
rate of cows on herd numbers is reflected. From a herd
size of 100 cows at the start and 75% survival of heifers from birth to first calving, the herd numbers in year