The third trimester (months 6 to 9) of a beef cow’s pregnancy is the most important time during the feeding period. During the last 3 months of pregnancy, her requirement for energy increases by 34% from 12.6 Mcal/day at the end of the 6th month of pregnancy to 16.9 Mcal/day in her 9th month of pregnancy, see table 1. Most of this increase comes from the energy required for pregnancy which jumps from 1.4 Mcal/day at the end of the 6th month of pregnancy to 6.2 Mcal/day in the 9th month of pregnancy, an almost 450% increase in energy demand. These figures assume the pregnant cow is in good body condition (body condition score = 3) and sheltered from the cold and wind. Beef cows with a lower body condition score and/or being fed outdoors will have a higher energy demand than mentioned above.
| Months Pregnant | 4 | 5 | 6 | 7 | 8 | 9 |
| Net Energy Required, Mcal/Day | ||||||
| Maintenance | 10.5 | 10.5 | 10.5 | 10.5 | 10.5 | 10.5 |
| Pregnancy | 0.4 | 0.7 | 1.4 | 2.4 | 4 | 6.2 |
| Lactation | 1.6 | 1.1 | 0.7 | 0.5 | 0.3 | 0.2 |
| Total | 12.4 | 12.3 | 12.6 | 13.4 | 14.8 | 16.9 |
It is critical that the energy demands described above are met by providing her with a diet that meet these requirements. Failure to provide sufficient energy in the diet at this time can lead to weight loss, challenges at calving, poor colostrum and milk production, reduced calf thriftiness and a longer post natal interval before the cow will be ready to breed.
During the last 3 months of pregnancy beef producers should be feeding their “best” quality hay or other forage type. Forage sampling is critical to determine what “best” quality is. Ideally forage sampling should be completed well in advance of feeding so the quality of the feed to be fed can be determined and any supplementary feed, if needed, can be readied. Lower quality forage should be targeted to those beef cows in the 2nd trimester, where their energy requirements are lower. The importance of portioning out your hay supply based on its quality is a very important management tool. Doing so ensures that the lower quality forage goes to those animals that have lower feed requirements and the higher quality forage is reserved for those animals that really need it. Assuming that there is enough of both lower and high-quality forage available to meet herd demands, this will have the effect of keeping costs to a minimum.
A 50-cow beef herd (assuming average 1,200 lbs beef cow) will consume approximately 108,000 lbs. of “lower” quality hay in the 2nd trimester and an equal amount of high-quality hay in the 3rd trimester. Knowing what’s in your hay inventory at the start of the winter-feeding period and periodically throughout the winter-feeding season will ensure your hay inventory is managed correctly. In this simple example the hay supply is split 50:50 between “lower” quality and “higher” quality. In this situation and assuming the “higher” quality hay is sufficient to meet the herd’s requirements in the last trimester, no additional feed should be required, so the feed cost to the producer is simply the cost to produce the forage and the labour to feed it out.
However, in the event that the hay inventory splits 60:40 in favour of the lower quality hay, the effect will be to force the producer to feed some lower quality hay for some period of time of the 3rd trimester. To offset any nutritional deficiencies, supplementary feed may be required, which adds to the winter feed costs.
For those feeding their cows outdoors, the effect of climate must be considered. As a rule, beef cows acclimatize to the cold very well. As fall turns to winter and the days get colder, beef cows grow long hair and lay down a layer of adipose tissue beneath the skin. Long hair traps more air close to the body, which acts like insulation. Maintenance requirements increase by 1% for every degree below 0° C. An 8th month pregnant beef cow at 0° C requires 14.8 Mcal/day of energy but at minus 20° C will require 17.76 Mcal/day of energy in the diet. Hay alone is unlikely to meet such an energy requirement and so some supplementary feeding will be required. The energy demands described are based on a beef cow that is kept dry and sheltered from the wind. If the coat is wet or muddy, maintenance requirements increase by 2% for every degree below 15° C. So even at 0° C, a muddy or wet coated animal can have a significant increase in energy requirement.
Body condition score plays a highly important role in the energy requirements of late pregnant cows. To minimise calving difficulties and maximise both calf performance and subsequent breeding performance, cows should calf down at a body condition score of 3. A significant body of research has shown the negative effects of cows calving in poor body condition (i.e., less than a score of 2.5) or being overfat at calving. Correcting for body condition score should begin at the start of the winter-feeding season and cows should be monitored for body condition on a regular basis over the feeding period. Any cows that show signs of losing weight should be removed from the main herd to a smaller, higher feed group. This is especially important for 1st calf heifers,who should not be fed with mature cows, and for mature cows in their 2nd pregnancy.
Cows that become thin during the 3rd trimester present a feeding challenge. If these cows can be identified as getting thin early in the third trimester, (i.e., month 7 of pregnancy), there may be an opportunity to return them to good body condition with a modest amount of supplementary feeding, which will not be detrimental to calving. Cows that get thin later in the 3rd trimester cannot be fed a high enough energy diet to restore them to good body condition without causing issues at calving. Very high energy diets fed to cows in late pregnancy can result in the deposition of fat in the birth canal, leading to calving difficulties. In this case, it is best to feed a diet sufficiently high energy and accept the fact that the cow will calve down in less than ideal body condition.
The protein requirements of late pregnancy cows are around 690 grams per day of metabolizable protein, see Table 2. This rate is about 24% higher than seen with cows in their 2nd trimester. Fortunately, supplying adequate protein to meet demand is relatively simple. As an example, a 10% crude protein mixed hay (TDN = 60%) will provide an excess of protein for a beef cow in her 2nd trimester and meet her protein requirements late in her 3rd trimester. Where protein requirements are inadequate, this is easily correctly through supplementation with high protein by-products (e.g., corn distillers’ grains). As with energy, the protein supplied in the diet should be accounted for through a forage test and as described above, forages with lower protein levels should be targeted to those cows in mid pregnancy whereas higher protein forages should be targeted to those cows in later pregnancy.
Table 1. Protein Requirements of a Pregnant Beef Cow in 2nd and 3rd trimester, (1,210 lbs., Body Condition Score = 3).
| Months Pregnant | 4 | 5 | 6 | 7 | 8 | 9 |
| Metabolizable Protein Required, Grams/Day | ||||||
| Maintenance | 432 | 432 | 432 | 432 | 432 | 432 |
| Pregnancy | 14 | 27 | 50 | 88 | 152 | 251 |
| Lactation | 114 | 78 | 53 | 35 | 23 | 15 |
| Total | 560 | 536 | 534 | 555 | 607 | 697 |
Forage analysis is necessary to determine the mineral requirements of late pregnancy cows. Calcium and Phosphorus requirements increase by 21% and 14% respectively from 6th month of pregnancy to the 9th month of pregnancy. However, the requirement for calcium and phosphorous increases by another 25% by the 2nd month post calving, driven by the need for these minerals in milk production. Producers should consult with their nutritionist to determine the correct mineral to be fed that matches their unique requirements and supply.
Meeting the nutritional requirements of late pregnancy beef cows is critical so that cow’s calf in good body condition score, deliver a healthy calf that thrives and enables those cows to go back in calf as soon as practical. The key to achieving this is timely forage analysis, an understanding of the on-farm forage inventory accounting for the results of the forage analysis, the distribution of the forage to beef cows at the correct stage of pregnancy to meet both protein and energy requirements and, where necessary, providing supplementary feeding to balance out any deficiencies.
References:
Byrne, J., 2022. Preparing your Beef Cattle for Winter. Virtual Beef. Ontario Ministry of Agriculture, Food and Rural Affairs, Preparing your beef cattle for winter – Dairy and Beef Cattle Production (wordpress.com)
Byrne, J., 2021. Winter Management of Beef Cattle. Virtual Beef. Ontario Ministry of Agriculture, Food and Rural Affairs. Winter Management of Beef Cattle Virtual Beef December 2021 – Dairy and Beef Cattle Production (wordpress.com)
Managing forage supplies for beef cattle. 2022. Ontario Ministry of Agriculture Food and Rural Affairs. Managing forage supplies for beef cattle | ontario.ca
McKinnon, J., Feeding the Pregnant Beef Cow. 2022. The Beef Magazine, Canadian Cattlemen, Feeding the pregnant beef cow – Canadian Cattlemen
Olsen, K., Are Your Cows Ready for the Last Trimester of Pregnancy? 2022. South Dakota State University Extension, Are Your Cows Ready for the Last Trimester of Pregnancy? (sdstate.edu)
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