In fall I visit several different cow herds going into winter. One of the first herds I saw this year was a 300 Angus-Simmental cows grazing drought-stricken pasture.
I understood from the producer that his herd was nearly four months pregnant and the calves were going to be weaned about a month earlier than in the past. I felt the cow herd lacked body condition, despite most of its mature cows having big bellies (perfect for consuming lots of coarse prairie grass) and the surprising good shape of their calves — one or two, I would even call plump. As I was about to leave, I talked to the producer and he agreed with many of my observations; leading to the consequence that he move his cow herd onto better quality pasture to improve their overall body condition. After a month or so, he would move them closer to home in order to maintain them on a well-balanced feeding program until the calving season.
A lifetime impact
A concept coined as “fetal programming” theorizes that the lifetime performance of calves can be positively or negatively affected by the soundness of cow nutrition during all gestation trimester stages (from conception to calving).
This concept further specifies that even though 75 per cent of fetal growth occurs during the last trimester, early and mid-term nutrition of the gestating cow (and her fetus) are just as important. This is when maximum growth and vascularization of the placenta takes place, which regulates fetal growth. In addition, major fetal organs such as the liver, lungs, brain and kidneys become significantly developed compared to “fetal growth” of muscle and fat tissue, which occur in late gestation.
Although scientific trial results are mixed, the concept makes logical sense.
For example, the University of Wyoming (2010) evaluated the growth performance and organ development of calves born to cows for which nutrient intake was restricted for the first 83 days of early gestation. It was later resumed to 100 per cent of NRC levels until calving. Data showed: birth, weaning and finishing weights were not significantly affected, but at slaughter the weights of heart and lungs were reduced. Months later, another U of W study (2010) showed that steers born to cow grazing low-quality pastures (six per cent protein) for 60 days during mid-gestation had lower weaning weights and finished carcass weights.
These field trials are good reminders that beef producers should first take a visual assessment of each cow after weaning calves in autumn/ winter and assign a body condition score or BCS, which ranges from 1 = emaciated, 5-6 = optimum and 9 = obese. This score represents her level of body fat covering or energy status upon which she can draw during periods of dietary energy deficiencies. Mature cows should maintain or achieve a BCS of 5-6 from the beginning of winter until the day of calving. Replacement heifers should calve out at a little better BCS of 6.0.
As well, producers should never allow any cow to lose optimum body condition once winter temperatures fall and she needs to elevate her total energy requirements just to keep warm. More university and extension environmental study on beef cattle has come up with a linear cold weather rule of thumb as follows: for every 1 C drop in temperature below 0 C, the beef cows’ TDN energy maintenance requirement increases by about two per cent.
This calculation is an estimate and is based upon effective air temperatures. Producers can also use windchill temperatures without adjustments, when cows have little shelter. This means if there is an early morning windchill temperature of -25 C, then there is an increase of about 50 per cent in the cows’ basic dietary energy needs.
A simple ration
Given these facts, I often formulate overwintering diets for gestating beef cows on a very simple plane of nutrition.
For example, I recommend the best candidates to be fed straw as an economical forage in over-winter diets are mature early to mid-gestation beef cows. These pregnant cows have nutrient requirements of about 53-55 per cent TDN dietary energy, nine to 10 per cent protein, 0.25 per cent calcium, 0.20 per cent phosphorus and a good compliment of essential trace minerals and vitamins.
I put together a TMR diet that consists of: 20 pounds cereal straw, eight pounds barley, 1.5 pounds of a 32 per cent beef protein supplement and a commercial 2:1 cattle mineral fed at 0.25 lbs per head. Total cost of my diet is about $ 1.50-1.75 per cow daily.
I suggest feeding this straw-based diet until the last three months before calving, then switch to higher-quality forages such as mixed hay, possibly supplemented with more protein supplement and grain. At any time when cold weather becomes a dietary factor, I would increase the energy-enriching grain portion of the diet substantially.