I got this in the Hereford Enews email. It is Dr Hall writing about winter calving increasing calving difficulties....
"John Hall, Virginia Tech Department of Animal & Poultry Science
No sooner did I mention that the winter has been warmer, yet still stressful, than we were hit with almost 30 days of below average temperatures. Producers have often speculated 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.
The research
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 have noted that lambs from winter-shorn ewes are heavier at birth and are more vigorous than lambs from unshorn ewes.
Properly conducted research on weather effects on birth weight in cattle is limited. However, in the 1990s researchers from Nebraska conducted a study on birth weight and dystocia as affected by winter weather. For six years, data was collected from March-calving heifers of similar breeding that were all bred AI to the same calving-ease Angus bull. 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. Researchers found almost a 1:1 relationship between decrease in average temperature and increase in calf birth weight. For every 1-degree (Fahrenheit) drop in average or wind chill temperature there was roughly a 1-lb. increase in calf birth weight.
One of the years did not seem to fit. The winter of '95-'96 was relatively warm, but calf weights and dystocia increased. When researchers reviewed the weather data, they found that January of '96 was much colder than average. Therefore, they concluded that this one month in late gestation was sufficient enough 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 ensures that essential organs (brain, heart, liver) continue to function normally even in cold weather. As a result of this shift in blood flow, there may be greater blood flow and nutrients reaching the fetus. However, short-term cold exposure in women has actually been shown to decrease blood flow to the fetus.
Another theory is that voluntary feed intake is increased in dams exposed to cold. While ruminants can compensate to some extent by increasing intake during cold weather, this 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.
Management of heavier calves
For mature cows, the increase in birth weight of 5-8 lb. caused by cold weather results in little increase in dystocia. However, producers should be aware of increased birth weights and the potential for problems. If extremely large birth weights occur, then cows should be checked more often.
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 30-45 minutes. Make sure that heifers are well-dilated and be sure the calf is of 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 in the following breeding season."
I doubt this will change ANYBODY's opinions one way or another; but it is food for thought.
