The Cow-Calf Manager

Livestock Update, March 2007

John B. Hall, Extension Animal Scientist, Beef, Virginia Tech..

Cold Weather May Increase Calf Birth Weights

No sooner did I mention that the winter has been warmer, yet still stressful, that we were hit with almost 30 days of below average temperatures.  Producers have often felt that calves are bigger after cold winters.  Is this really true?  Often it is hard to compare one year to the next as bulls and nutrition change which can affect calf birth weight.  However, several studies indicate that exposure of dams to cold conditions can result in increased birth weights of offspring.

What does the research indicate?
Some of the early work in this area was with sheep.  Shearing ewes before they lamb is a common practice to reduce soiling of wool, make it easier for lambs to nurse, and reduce space needed in lambing sheds.  Researchers noted that lambs from winter-shorn ewes were heavier at birth and were more vigorous than lambs from unshorn ewes.

Properly conducted research on weather effects on birth weight in cattle is limited.  However, in the 1990’s researchers from Nebraska conducted a study on birth weight and dystocia as impacted by winter weather.  During six years, data was collected from March calving heifers of similar breeding that were all bred AI to the same calving-ease Angus bull (same bull used all years).  The research group kept track of average and wind chill temperatures from December – February of each year.  All calves were weighed and any heifers requiring assistance were noted.  Birth weights were heaviest and calving difficulty greater in the coldest years (Table 1).  They found almost a 1:1 relationship between decrease in average temperature and increase in birth weight of the calf.  For every one degree drop in average or wind chill temperature there was roughly a one pound increase in calf birth weight. 

One of the years did not seem to fit.  The winter of 1995-1996 was relatively warm, but calf weights and dystocia were increased.  When the researchers reviewed the weather data, they found that January ’95-‘96 was much colder than average.  Therefore, they concluded that this one month in late gestation was sufficient to affect birth weight.

Why does birth weight increase?
One theory behind increased birth weights of offspring from cold exposed dams relates to blood flow.  It is well established that when exposed to extremely cold temperatures, mammals shift blood flow from extremities and skin to major internal organs.  This survival mechanism assures that essential organs (brain, heart, liver) continue to function normally even in cold weather.  As a result of this shift in blood flow, it may result in greater blood flow, and therefore, nutrients to the fetus. However, short-term cold exposure in women actually decreased blood flow to the fetus.  Another theory was that voluntary feed intake was increased in dams exposed to cold.  While ruminants can compensate to some extent by increasing intake during cold weather, it probably does not explain all the increase in birth weight.

More recently, several groups have found metabolic changes in cold stressed ewes.  These metabolic changes result in increased availability of glucose, IGF-I, and IGF-II to the fetus.  Glucose and the two insulin-like growth factors (IGF) are important regulators of fetal growth.  Some studies also indicate an increase in brown adipose tissue, the fat that is used in the first days of life to produce heat, in newborns.  Increased size, enhanced energy availability, and greater brown adipose tissue are positively correlated to newborn calf survival during cold weather.

Table 1. Effects of winter temperatures over six years on calf birth weight and calving difficulty. (Deutscher et al., 1999)

a Winters ranked from coldest to warmest. High and low temperatures for December, January and February were averaged.
b Average wind speed x average temperature for 3 months.
c Birth weight adjusted for calf sex.
d Calving difficulty with scores 3 to 5 on a 1 to 5 scoring system.
efg Means within category differ (P < .05).

Management of heavier calves
For mature cows, the increase in birth weight of 5 to 8 lbs. caused by cold weather will result in little increase in dystocia.  However, producers should be aware of increased birth weights and be observant for potential problems.  If extremely large birth weights occur then frequency of observation of cattle should be increased.

First calf heifers will need more observation and close attention this calving season.  Producers should be prepared to check animals or render assistance if heifers appear to be struggling or have been in Stage 2 labor (hard pushing) for ½ hour to 45 minutes.  Make sure that heifers are well dilated and be sure the calf is a deliverable size. Don’t be afraid to call the veterinarian if needed; the value of a live calf is worth the vet call.  But remember, if you have worked half the night trying to pull the calf, don’t expect the vet to perform a miracle and deliver a live calf…..call early.

Under no circumstances should producers reduce feed levels or amount of energy in late gestation diets in an attempt to reduce calf birth weight.  Reducing energy intake will cause weak calves and thin cows.  In addition, thin cows will have poor conception rates during the breeding season.

References

Butt, E.A., S. Pearce, T. Stephenson and M.E. Symonds. 2004. Maternal cold exposure from mid to late gestation results in increased adipose insulin like growth factor (IGF)-I mRNA expression and
heavier birth weight with no effect on fat mass. Endocrine Abstracts 8:21

Deutscher G., D. Colburn and R. Davis. 1999. Climate Affects Calf Birth Weights and Calving Difficulty. UNL Beef Cattle Report, University of Nebraska Cooperative Extension MP 71  http://ianrpubs.unl.edu/beef/report/mp71-03.htm

Kimura, Y., K. Okamura, T. Watanabe, T. Takahashi, I. Haga and A. Yajima. 1998. The Effect of Cold Stress on Uterine Artery Blood Flow Velocity Waveforms in Late Pregnant Women with and without Preeclampsia”. Tohoku J. Exp. Med. 186: 71-77

Revella, D. K., S.F. Main, B.H. Breier, Y.H. Cottam, M. Hennies and S.N. McCutcheon. 2000. Metabolic responses to mid-pregnancy shearing that are associated with a selective increase in the birth weight of twin lambs Dom. Endo. 18 409-422

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