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Biological Cycle and Nutrient Requirements

So the biological cycle for a beef cow can be broken up into...

The postpartum period, that is about 80 days. And so, by 80 days, you enter the first trimester. So that means, at this point, the cow should be pregnant. It is much more imperative that beef cows get pregnant within 80 days than it would be a dairy cow that has continuous calving throughout the year. If you have a spring-calving operation and you lose ten days each year, can you image what would happen? So, suppose each year you want to calf in April and you lose 15 days. In a couple of years, you are behind by two months, right? And now you are summer-calving. And eventually, you are fall-calving. And maybe in fifteen years, you are winter-calving. Would winter-calving be good in Burns, Oregon? Probably not. It is imperative that these cows conceive by, at the latest, 80 days after parturition. Then, we have the three trimesters divided into 95 day periods. And so you end up with a yearly calving interval.

Beef producers are concerned or not concerned as much with maximum production as dairy producers. Beef cows should be fed for optimal production, not maximum production. And so there is a difference. With dairy cows, maximum production is optimal production. But, it may not be the same for beef cows. The early postpartum period, the first 80 days after calving, this is the period of highest nutrient requirements, primarily due to milk yield. You can expect milk yields for beef cows from about 10 to 20 pounds per day. During this period, they must be in adequate body condition, the same as a dairy cow. But it is even more imperative because if cows are in poor body condition, they will not cycle and that means they will not conceive within 80 days after parturition. And because beef cows are so dependant upon forages, that means that forage quality during this early postpartum period is critical.

Once the cow is pregnant, she enters the first trimester. Milk yield is declining at this point. Even though she is pregnant, the conceptus requires relatively few nutrients.

During the second trimester, the calf is weaned. So, now she is not lactating. She is 100, 190 days pregnant. The fetus still is not growing rapidly, so she has very low nutrient requirements. And during this period, if the cow is in poor body condition, this is the time to put fat back on her back.

During the third trimester, nutrient requirements start to increase again. So beef cows, during these last 95 days before parturition, will gain somewhere around a pound per day and this is mainly in conceptus weight not in body fat. It is difficult to change body condition during this period because you have a relatively short period of time. But thin cows are at increased risk for dystocia, calf morbidity, calf mortality and also likely to give less milk during the subsequent lactation. Again, forage quality becomes critical.

If you look at the critical points of time for forage quality, they are in the early postpartum period, or the first 80 days of lactation, and also during the third trimester, so about the last three months before the cow calves.

The majority of the time, energy is considered first limiting or you consider it first in balancing diets. I showed you this in the dairy cow section, but maintenance is equal to some coefficient times body weight. What happens to maintenance energy requirements as body weight increases? They increase. Do you think there is some optimal body weight for a cow that is on range? Do beef producers want 1,700 pound, 2,000 pound Holstein cows or equivalent to Holstein cows out grazing range? No, because their maintenance requirements are so high and there is little energy to begin with and so you want the medium size, small cow to reduce maintenance energy requirements. Also, remember the physical activity of these cows and how far they are walking each day. You can expect to see a 30% to 50% increase on range cows versus confined cows. So a 30% to 50% increase in maintenance requirements.

On dormant range, in the fall, winter you can usually expect a protein deficiency. It will get somewhere, or it can get below, 7% crude protein on a dry matter basis. When the total diet gets below 7% crude protein, you have insufficient ammonia in the rumen for microbial growth. So if the microbes stop growing or aren’t breaking down feed as efficient or as fast as they were, what will happen to intake? It will also go down. And so you reduce digestion in the rumen, it leads to reduced passage rate and eventually reduced intake. Since we are in the protein section, I will mention protein supplementation. We will talk about positive associative effects between if you supplement a cow with protein. There are some other positive effects not only in just giving her more protein.

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