This is a follow on post from last week where we covered the genetic effects of calving difficulty in dairy cattle. As there are many, many factors that can contribute to a cow experiencing a difficult calving, also known as dystocia. This post will cover the non-genetic factors such as nutrition that affect dystocia.
The main causes of dystocia are outlined here and as we said, this post is going to focus on the non-genetic effects (e.g. nutrition, weather) on feto-maternal disproportion.
Key Points:
Nutrition in early- and mid-pregnancy had no effect on calf birthweight.
Restricting feed in late-pregnancy decreased the birth weight of the calf but also decreased the liveweight of the cow.
Severely cold winter temperatures in the final few months of pregnancy have been associated with heavier birthweights and more calving difficulty.
When we were hunting out studies to include here, the majority were looking at nutrition in beef cattle not dairy. We have included studies from both.
The growth of the calf from conception to birth follows an exponential growth curve; this means that the greatest absolute increase in weight occurs in the last (third) trimester, yet the greatest relative increase in weight occurs in the first trimester of pregnancy. This is shown in the graph below. Therefore, it could be expected that changes to the nutrient supply to the foetal calf either in the early stages or late stages of pregnancy may have an impact on birth weight or the chance of calving difficulty occurring.
Pre-pregnancy
Nutrition
The proportion of crossbred beef heifers that required assistance at calving was not different if they were reared to 50% (277 kg) or to 55% mature weight (308 kg) at breeding.
This study was completed in the USA and the proportion of heifers requiring assistance was high (31.3% for the 277 kg vs 24.7% for the 308 kg heifers). A survey of NZ beef farmers reported that the incidence of assisted calvings was much lower at 7% in first-calving 2-year-old heifers. Although, the percentage of assisted calvings reached 100% in some herds.
A high weight gain compared with a moderate weight gain in the 10 days before breeding may be associated with a reduction in calf birthweight. Angus heifers that were fed to gain a moderate weight in the 10 days prior to breeding followed by a loss in weight in the first few months of pregnancy had heavier calves than heifers that had a high weight gain before breeding followed by a loss in weight in the first few months of pregnancy, provided the heifers were the same weight at calving.
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Other
Calves that are produced by in vitro methods (e.g. embryo transfer; ET) had longer gestation lengths than calves produced by artificial insemination (AI). Additionally the cows that carried the ET calf were more likely to experience calving difficulty (9/41) than cows that carried an AI calf (1/42); however, there were low numbers of cows in the study so the results should be interpreted carefully.
In the same study, there was no difference in birthweight of calves produced by ET or AI, however, other studies have reported that calves produced by ET are heavier at birth than those that were produced by ET. The longer gestation and potential heavier birthweights are likely to increase the chances of a difficult calving.
Early- and Mid- pregnancy
Little is known about the effect of feeding levels in early pregnancy on the birth weight and calving ease in dairy cows. The nutritional requirements of the foetus are very low in early pregnancy, therefore, changing nutrition in early pregnancy is unlikely to influence dystocia.
There were no differences found with changing feeding levels during mid-pregnancy which is unsurprising given that mid-pregnancy has the lowest absolute and lowest relative increase in feotal weight as shown in the graph above.
Late-pregnancy
Overfeeding
It appears that if nutrition in late-pregnancy has an influence on calving difficulty, then the two extremes (severe underfeeding or severe overfeeding) was associated with an increased risk of calving difficulty.
For example, in first-calving Holstein-Friesian heifers, those that conceived later (delayed bred) had an increased risk of dystocia occurring. The authors hypothesized that this may be due to increased body fat of the delayed bred heifers compared with the heifers that conceived to their first breeding.
One study on dairy cattle in Ireland compared feeding grass silage only or grass silage and 5kg of concentrates in the last month before calving and reported that there were no differences in calving difficulty or calf birthweight between the 2 groups.
Angus-Simmental heifers fed a diet containing dried distiller grains that was excess to requirements before calving had longer gestations, heavier calves and a greater incidence of dystocia than heifers that were fed a corn-silage based diet to meet their feed requirements. This study was based around diet differences and how it was related to calving parameters and is not particularly transferable to pasture-fed dairy cattle.
Based on these studies and others it appears that overfeeding during the last few months of pregnancy, to the point that cow body fat is increased, may be associated with fat deposition in the birth canal that can cause dystocia.
Underfeeding
Underfeeding in the last few months of pregnancy has been reported to have more of an effect on calf birthweight for heifers than older cows. Authors have hypothesized that this is because there was more competition for energy from pregnancy and growth of heifers compared with older cows that are fully grown (or close to being full-grown!).
Severe underfeeding of heifers in the last months of pregnancy has been shown in some studies to reduce birthweight of calves, but not to consistently reduce the chance of dystocia. In some studies, there was actually an increase in dystocia occurring as the development of the heifer was restricted, so both the calf size and the dam size was reduced.
Beef heifers kept on a low allowance for the last few months of pregnancy took longer to return to oestrus and to conceive than heifers that were kept on a high nutritional allowance.
Based on these studies, there is the potential that attempting to limit calf birthweight by underfeeding cows during late pregnancy may also result in compromised performance of the cow in early lactation.
Other
Interestingly, temperature in the final few months of pregnancy has been associated with calf birthweights and calving difficulty scores.
A study on crossbred beef heifers reported that heavier birthweights and more calving difficulty was expected in the spring following severely cold winter temperatures. They hypothesized that in cold weather the blood flow to the uterus would increase, allowing for more nutrients to pass to the growing calf, causing an increase in birthweight.
Conclusions
Differences in birthweight have not always resulted in differences in the incidence of dystocia, primarily due to differences in liveweight of the heifer also induced by feeding regimens.
The ability of the farmer to regulate birth weights of calves through nutrition of the cows during pregnancy was limited by the ability of the cow to use her body reserves to buffer nutrient supply to the foetus.
The effects of nutrition on dystocia may be mediated by nutritional effects on the birthweight of the calf or size of the cow, which impact on the incidence of feto-maternal disproportion, or affect the intensity of the cow’s efforts during labour.
Variability in the incidence of dystocia in response to feeding level in the third trimester of pregnancy makes it difficult to make recommendations for the ideal nutrition of cows at this stage of gestation.
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