Animal Performance Under Drought: What Should Be Culled
Blaine E. Horn, Ph.D., Extension Educator, Johnson CountyDrought in the vocabulary of a range livestock producer is lack of sufficient precipitation
at the time of year in which plants are dependent upon the moisture for growth. As a
result, there is a significant decrease in forage production compared to long-term
averages. Because stocking rates are generally based on long-term forage production
averages, a drought results in there being a greater demand on the forage resource than it
can provide. In addition, drought not only reduces forage production, but also lowers
forage quality. Thus, not only do livestock have to range further to obtain their dry matter
forage needs, they also have to consume more of the forage to meet their nutrient needs.
Reducing livestock numbers in a planned systematic method when drought occurs can
alleviate losses in animal performance and prevent severe degradation of the range.
Plant Response to Drought:
Leaf and shoot growth slows as soil water becomes less available. Tillering in grasses is
also reduced. As drought conditions persist, leaves wilt, fold, become discolored, and
may eventually die. Even under mild water deficit conditions, there is a reduction in cell
wall formation, cell division, and protein synthesis. Plants adjust to low soil moisture by
shutting down and going dormant. This results in a reduction in the total amount of
above- and below-ground plant biomass that is produced.
During periods of drought plants utilize carbohydrates that were produced the previous
growing season and stored in the roots or crown of the plant. This drain on stored
carbohydrates results in loss of root vigor and fewer basal buds develop for next year’s
growth. Thus drought can result not only in a reduction in forage in the year of the
drought, but also in subsequent years; even if growing season moisture is adequate. The
philosophy that grazing can not hurt a drought stricken plant is erroneous. Good grazing
management is even more critical in times of drought. Plants need as much leaf material
present as possible in order to take advantage of any moisture that does occur to produce
energy for its roots and basal buds. The less energy the plant is able to provide its roots
the more vulnerable it becomes to the drought and to grazing.
Plants of arid and semi-arid regions have evolved to grow and reproduce when soil
moisture is sufficient. The time of year that water is generally available for plant growth
dictates the type of plants that grow; in Wyoming it is in the spring. As a result most of
our range grasses and forbs are cool season plants with the C3 photosynthetic pathway.
Theses plants begin growth when day time temperatures are consistently above 40
degrees and shut down and go semi-dormant when the temperature is constantly above 75
degrees. If soil moisture is not adequate during the peak growing months of April and
May, plant growth will be reduced. Summer precipitation; even that above average, will
not result in significant growth of cool season plants. However, if there is leaf material
present for photosynthesis to occur, energy will be provided to the plant’s roots and to its
developing basal buds for next spring’s growth.
Affect on Animal Performance:
Low forage production results in livestock having to range further to obtain necessary
amounts of dry matter to meet their nutritional needs. This increased travel results in the
expenditure of additional energy that may result in a loss in body condition for mature
animals and reduced gains in immature stock. Besides producing less forage, drought
stricken plants also contain lower concentrations of crude protein, energy, vitamin A, and
phosphorus. Thus, grazing animals need to consume a greater quantity of the forage to
obtain needed nutrients. However, if the forage is high in fiber (high NDF) livestock may
not be able to consume enough dry matter to meet their nutritional needs due to bulk
limitation.
If a cow or ewe is not able to obtain sufficient nutrients, particularly energy, she will lose
considerable weight. A loss in body condition of a lactating cow or ewe results in a
decrease in milk production and reproductive activity may cease. Body condition scores
(BSC) of a cow 4 to10 years of age needs to be above a 4 and for a ewe 3 to 7 years of
age above a 3 at time of birthing and upon entering the breeding season or they will have
lower milk production and fertility. The end result is light-weight calves and lambs and
open dams. Young cows and ewes need to be in a body condition above a 5 and a 3.5,
respectively, to be productive.
A reduction in energy intake by sheep as a result of drought can also adversely affect the
cross-sectional area of wool fibers by as much as 400%. This reduction in size weakens
the fiber resulting in “tender” wool or even a “break” in the wool. In addition, if their
dietary protein level drops below 80% of that required, production and quality of the wool
is negatively impacted.
A mature cow (1000 to 1400 lb) at peak lactation requires between 23 and 34 pounds of
dry matter per day depending on her weight and her milking ability. When dry she needs
only between 21 and 27 pounds per day. Remember the larger the cow the more dry
matter she requires in her diet. A mature ewe (110 to 198 lb) at peak lactation requires
between 4.6 and 7.0 pounds of dry matter per day but only between 2.2 and 3.1 pounds
per day for maintenance. Knowing livestock forage demand and the forage production
capability of the range resource is necessary in determining proper stocking rates; and in
times of drought, this knowledge can be crucial in helping to make destocking decisions.
Although removal of the calf or lamb will reduce the amount of dry matter forage needed
by the dam, under severe drought conditions available range forage may be at a level that
is below the minimum amount recommended to be left under good grazing management
during “normal” production years. If this is the case, removal of the entire herd from the
pasture would be recommended as further damage due to overgrazing will only
exacerbate the effects of the drought. Overgrazing the range in times of drought will
severely limit its ability to recover once the drought breaks, and as pointed out above, the
plants are not able to take advantage of what moisture does occur further weakening
them.
Removal of calves may not significantly extend the grazable period, especially under
severe drought conditions, as they consume only 15% to 25% of the forage consumed by
the dam. However, for sheep, the removal of the lambs may be a more plausible practice
as forage demand by the ewes may be reduced by over 50%. Early weaning of calves (4-
months of age) should be done mainly to improve cow condition for winter minimizing
the amount of protein supplement that is needed. A cow going into winter in good body
condition is easier and cheaper to maintain than one that is in poor condition. This would
also be true for ewes.
If the forage resource is inadequate to support the herd early in the grazing season, than
removal of cow-calf pairs would be recommended instead of early weaning (< 4-months
of age). Early weaned calves take special care and generally the market for them is not
good. Remember, 80% or more of range plant growth will have occurred by early June.
If plant growth is reduced due to drought, early weaning will only extend the grazable
period by a few weeks at most.
Destocking Strategies to Maintain Herd Performance:
Cull any cow or ewe that does not have an off-spring at side regardless of their age or
pregnancy status. They will not provide income in the current year unless sold and they
reduce the amount of available forage for dams with suckling young. For cow-calf
operations these cows usually comprise 5% to 15% of the cow herd. Replacement heifers
and ewe-lambs, whether bought or raised, should also be culled in this first round of
culling. These females will not provide income for over a year if kept and they require a
higher plane of nutrition to be productive. For cow herds removal of replacement heifers
can result in a further stocking rate reduction by as much as 15%. Stocker cattle, either
boughten or carried over from the previous year’s calf crop, should also be culled. They
will not gain well on low quality forage reducing the income received for pound of gain.
In addition, pastures used for stockers can then be made available for the cow herd.
Animals with structural (teeth, feet, legs, udders) or production (poor milkers, unthrifty
offspring) defects should be culled. In addition, cows that tend to calve late should be
culled. It is difficult to back up a late calving cow without special attention to her
nutritional needs. Use drought as an opportunity to tighten up the breeding season if
needed and to remove the least productive animals. This will improve the production
capability in the remaining herd.
If further culling is needed, determine whether the remaining cows are bred and cull any
open cows. If calves weigh 400 pounds or greater, wean and sell the calf and cow
separately. For calves weighing less than 400 pounds they along with the dam should be
marketed as pairs.
Further weaning of calves should be based on calf age and the need to improve cow
condition.
Additional culling of the herd needs to be based on the idea that those animals remaining
will produce offspring in the following years. Dams with the lowest rebreeding potential
should be culled. These would be thin (BCS 3 and under) middle-aged cows and thin to
moderate (BCS 4 and under) first and second calf cows. These cows will have lower
conception rates and wean lighter calves than cows in better body condition. Cows 8-
years old and older should be culled next. These cows will be the first to decline in
production potential in the future.
At this point, if any further culling of the herd is necessary it should be based on
uniformity; culling the smaller and larger end of the cows. By this time all that will be
left of the herd is 4- to 7-year-old cows and possibly their calves. If forage supplies are
not adequate to maintain this base herd through the remainder of the grazing season the
only feasible option is to liquidate the herd. Purchasing forage to maintain these
remaining cows cannot be justified economically. The long term survival of the ranching
enterprise and/or the economic security of the family could be in jeopardy if ill-advised
economic decisions are made.