As I write this column, 2020 is finally in the rear-view mirror. I am sure that I am not alone in hoping that 2021 brings better days and in particular an end to this pandemic that has had such a personal and economic impact on the lives of Canadians.
Despite this disruption to normal life, cow-calf operators are looking forward to spring and in particular to the start of a new calving season. However, before we get too excited, we should review the relationship between proper nutrition of the brood cow and her subsequent calving and breeding success. Specifically, we want to understand the impact of pregnancy on the nutrient requirements of mature cows and first-calf heifers and use this knowledge to develop late-gestation feeding programs.
To illustrate this relationship, let’s review some basic biological principles regarding fetal growth. First, remember that a typical bovine pregnancy lasts approximately 283 days or just over nine months. Veterinarians typically divide this gestation period into trimesters, each with distinct fetal development milestones. For example, in the first trimester there is significant system (i.e. nervous, circulatory), organ (heart, liver, GI tract), skeletal and muscular development, all culminating in a small but recognizable bovine fetus. As pregnancy progresses, the fetus continues to develop and increase in size with the largest increase in fetal weight (i.e. approximately 70 per cent) occurring during the last trimester.
During gestation, the dam supplies the fetus with all essential nutrients via transfer across the placenta. This transfer of nutrients increases dramatically during the last trimester to match fetal growth. The critical point is that the dam is the sole source of nutrients for the fetus. Her requirement for nutrients during the last trimester can increase significantly depending on the nutrient in question. The dam’s nutritional status — specifically the quantity and quality of her feed supply — reflects directly on her ability to supply these nutrients. If she is deficient in any aspect of her nutrition, there is potential for the fetus to be deficient or more likely the dam will rely on stored nutrients (i.e. body fat and protein stores) to meet fetal demand.
Fetal consequences of inadequate dam nutrition can include blindness (lack of vitamin A) and white muscle disease (selenium deficiency) as well as stunted growth. Furthermore, as we learn more about fetal programming, we are starting to recognize that inadequate dam nutrition, even at an early stage of development, can negatively affect post-natal health and growth of the calf.
For the pregnant cow, a diet that fails to meet late-gestation nutrient requirements will typically result in a loss of body weight. This situation can have serious consequences for her subsequent breeding success. For example, it is well established that cows that lose significant body weight in the last trimester take longer to cycle after calving. This has potential to extend the breeding and subsequent calving seasons or, depending on the length of the breeding season, increase the number of open cows. Also, keep in mind that cows in poor condition after calving will not only take longer to cycle but can also exhibit poor first-service conception rates. As you can see, there are some very good reasons to pay close attention to the feeding program of the pregnant beef cow, particularly at this time of the year when cold weather can further compound the situation.
To give you an idea of the magnitude of the changes in requirements of key nutrients as pregnancy progresses, consider a 1,400-pound cow in late pregnancy (i.e. eight months) not experiencing cold stress. According to Alberta Agriculture’s CowBytes program, this cow requires a diet that is 13 per cent more concentrated in energy (Total Digestible Nutrients) and 20 per cent more concentrated in protein relative to what she required in mid-gestation. Her requirement for essential minerals such as calcium and phosphorus increase by as much as 50 per cent.
The challenge for the cow-calf operator is to develop a feeding program that meets these requirements in an economic manner. While there are numerous approaches, in almost all cases they start with an understanding of the forage feeding program. Cows that have been fed good-quality grass or alfalfa/grass hay in mid-gestation may simply require an increase in the quantity of forage provided to meet late-gestation energy and protein needs. A similar situation exists with silage-based diets.
Where the issue gets more complicated is when the supply of high-quality forage is limited or in situations where one has no choice but to feed poorer-quality forage (i.e. cereal straw, slough hay, corn stover). In such situations, not only is it necessary to increase the quantity of feed supplied but it is also necessary to increase its quality. This typically involves providing supplemental energy and protein in the form of cereal grains (i.e. barley or oat), byproduct feeds (i.e. pea or lentil screenings) and/or specific protein supplements (i.e. canola meal).
Finally, don’t forget to supply a supplemental source of the macro (i.e. calcium, phosphorus, magnesium) and trace (i.e. copper, zinc, manganese) minerals, as well as the fat-soluble vitamins (i.e. vitamins A, D and E). Direct incorporation into a total mixed ration as per the recommendations on the mineral tag is the ideal way of ensuring adequate mineral intake.