THE EFFECTS OF WEATHER ON HAY PRODUCTION
J. K. Evans
Department of Agronomy
Even a slight amount of rain on curing
hay can cause serious losses of feed quality. The losses occur because
much of the nutrition in the plant is water soluble and can be removed
by leaching. Studies have shown that rainfall of only 0.05 inch on hay
which is partially cured is sufficient to cause leaching losses. Increasing
amounts of rain cause greater losses by leaching and also by knocking off
the leaves, which contain much of the protein in legume hay. The Agricultural
Research Service has produced some data from experiments in Maryland which
show what the magnitude of these losses can be for alfalfa. The results
are shown in Table 1.
Table 1. Effect of rain on cured hay
1Rainfall amounts up to one inch.
||% leaves retained
||% protein retained
||% total digestible nutrients
||% decline in milk production
|Field Cured, no rain
|Field Cured, rain1
Throughout the hay-harvesting season,
rain in Kentucky is usually caused by either the passage of a front or
by daytime convective showers. In the first case, a cool front will move
through the state and be accompanied by a band of showers and thundershowers.
This may occur at intervals of 3 to 4 days, or more than a week may pass
without the passage of a front. Fronts are usually followed by clearing,
cooler weather. High pressure centers which move across the country behind
the fronts generally bring sunny skies, low humidity, and several days
of good drying weather. The passage of a cold front, then, is a signal
to the farmer that he can go ahead and cut hay. Some signs which indicate
the frontal passage are: a shift of the wind to the west, clearing skies,
lowering humidity, and a rapidly rising barometer.
While high pressure centers move progressively
across the country much like cold fronts, they will sometimes become stationary
for several days and bring persistent fair weather. These times are usually
extended dry periods with ample sunshine, light winds and low humidity.
This is the ideal opportunity to cut hay.
Often in the summer months high pressure
centers will become stationary along the southeastern coast of the U.S.,
and this causes a rather unsettled weather pattern in Kentucky. Southerly
winds on the west side of the high pressure center will bring moisture
from the Gulf of Mexico up into the Mississippi and Ohio Valleys. When
the moist air is heated during the day it rises until clouds form and finally
develop into showers. These are generally small showers which last less
than an hour but can bring a half-inch or more rain to a farm. This particular
weather pattern can continue for several days. Some farms may not receive
any rain, but anyone who cuts hay is taking a risk that one of the showers
may move across his field. The weather forecast which is issued each day
includes a "probability of precipitation," which is an estimate of that
risk of getting a hay crop wet.
Since we cannot expect long periods
of dry weather here in Kentucky, it's necessary to find a suitable period
to harvest hay and store it in the least time possible. This means reducing
the drying time to shorten the exposure to possible rain damage. The time
required to dry a crop from its initial moisture content down to about
15% moisture is determined by a number of factors, such as plant species,
density of the crop, soil moisture, and the way the crop is handled. For
any given situation, a combination of weather factors can cause the necessary
drying time. to vary from a day to over a week of rain-free weather.
Studies of the weather factors which
are related to the drying rate show the most important ones should be considered
in this order:
The sun provides the energy required
to evaporate water from the plants. During June and July, when days are
long, we have about 15 hours of sunlight between sunrise and sunset. In
October, when the last cutting is made, the hours of sunlight are reduced
to about 101/2. The lower angle of the sun during autumn also reduces the
amount of available sunshine. When selecting a field to be used for hay,
it is worth remembering that slopes facing south receive more of the sun's
energy, and there drying should be faster.
The air which is moving across the
top of the drying hay crop must be able to absorb the water which is evaporating
and mix it with the rest of the atmosphere. In this regard, air behaves
much like a sponge or a mop. Sunny, warm days have the effect of lowering
the relative humidity of the air and thereby increasing its ability to
absorb water while also increasing the rate at which water is driven from
Since most drying takes place during
the daylight hours, wind speed is an important factor during that time.
Air next to the crop surface would soon become saturated under calm conditions
and be unable to absorb additional water, so a certain amount of wind is
necessary to replace it with drier air. Brisk winds and fluffy, porous
windrows are distinct aids in increasing drying.
Heavy dews may also delay drying time.
Normally, heavy dews occur on clear nights when the earth, cooling, radiates
its heat back to the sky. As the surface temperature drops to the dew point,
water vapor in the air is deposited on the leaves and stems of the hay.
At sunrise, the energy which would otherwise be used to dry the hay must
be used to first evaporate the dew. Related to the moisture content of
the soil, heavier dews usually occur for several days after a good soaking
rain, then decrease in intensity as dryness continues.
A rain just before cutting may slow
the drying rate by keeping the bottom part of the windrow moist even though
,skies are sunny and the humidity has lowered. Little can be done about
this condition since cutting during fair weather right after a period of
rain is an ideal strategy. However, an anticipation of slower drying and
an additional turning or two may prevent this condition from becoming a
An early, heavy crop which is less
porous in the windrow will take longer to dry since it is difficult for
the air to move through it. Also, early heavy crops usually have a higher
initial moisture content and more water must be removed from them than
from later, more mature cuttings, which may have a higher fiber content.
The rate of drying depends on the time
of the year. Allow more days during the fall. The following table shows
the average daily evaporation for each month:
||Average Evaporation (inches/day)
The use of hay conditioners to crush
the stems of the plants as they are cut has been a very effective method
of reducing the period of time the crop is on the ground and exposed to
damage from rain. In some cases the time has been reduced to as much as
half that of conventionally-mowed hay under identical weather conditions.
Conventionally-mowed grass or alfalfa of average density and moisture content
will require about the same amount of time to cure as it would take to
evaporate about an inc@ of water from an open- metal watering trough. Conditioned
hay has required only about as much time as would be needed to evaporate
a half-inch of water from that same trough.
Haymaking Weather Tips
• Be Weather wise. Listen closely to
the extended weather outlook and the daily hay cutting advisory in addition
to the forecast.
•Try to cut your hay just after a cold
front passes in order to have the longest period of dry weather for the
crop to cure.
•Use a hay conditioner, if one is available,
to help speed up the drying rate and reduce the time the hay is left on
•Put the hay into a porous, fluffy
windrow so the air can move through it easily.
•If you turn a hay swath, do so when
there is still dew on it in order to reduce the number of leaves lost by
shattering. Try to avoid touching it during the middle of the day when
the leaves are brittle and easily knocked off
Showers accompanying the cold front make this time unsuitable for cutting
hay. Wait for definite signs the front has passed and a clearing trend
High pressure usually persists for several consecutive days with clear
skies and ideal weather to harvest hay.
High pressure along the Atlantic Coast brings humid air from the Gulf
Region into Kentucky. Listen to the latest forecast and probability of
precipitation to determine the risk of showers developing.