By Kirsten Nickles
The nutritional requirements for beef cows change daily throughout their annual production cycle. The frequent change in requirements is caused by varying stages of production and environmental factors that affect the cow’s behavior and energy use. To give an example, a spring calving beef cow gestating throughout winter will have energy requirements for maintenance and gestation, and there may be further requirements for cold stress if winter climatic conditions place the cow outside her zone of thermal comfort. To appreciate how great the total net energy cost of a beef cow can be we have included the net energy requirements in Mcal/day throughout the annual production cycle of a mature 1200 lb Angus cow with a peak milk yield (PMY) of 18 pounds (Figure 1). We included the requirements for maintenance, lactation, and gestation and assume this all occurs without any cold or heat stress on the cow. It is noteworthy to consider that thermal stress can elevate the requirement for maintenance substantially.
Many beef cattle producers would agree that the nutritional requirements of the cow are greatest when peak milk yield is achieved. During this time, the cow is expected to lactate and achieve peak milk by 40-50 days after calving, undergo uterine involution (the structural and functional regression of the uterus back to an adequate size and status capable of supporting a new pregnancy), return to estrous, and finally become pregnant. Peak milk yield is approximately 18 lbs/day for an Angus cow, 20 lbs/day for Charolais, 15 lbs/day for Hereford, and 26 lbs/day for a Simmental cow.
At the cow’s greatest net energy requirement, beef producers must provide sufficient energy as forage or concentrates to allow the cow to achieve her peak milk yield and re-breed by 80-90 days after calving. Meeting the cow’s energy requirements ensures that the cow maintains a 365 day calving interval and maximizes calf growth up to weaning. Providing your best quality and quantity of forage during the spring months helps to meet the energy requirements for the cow’s peak milk yield. Failure to meet the energy requirement in early to peak lactation will cause the cow to utilize her body tissues for energy and other nutrients. A decrease in one body condition score (scale 1-9) is the equivalent of losing approximately 80 lbs (35 kg) of body weight for an Angus cow, and current recommendations are that the cow should not lose more than 1 condition score from parturition to peak milk yield. The cow can be assisted by providing her with supplemental grain if the forage available is poor in nutrients and energy, or the quantity of pasture available is limiting.
The cow’s energy requirements will gradually decrease as lactation progresses. Breeding cows at 80-90 days after calving places further energy requirements on her in that the cow has to maintain a pregnancy and grow a fetus. Although the nutrient requirements for early pregnancy are barely above maintenance, the cow continues to produce milk for the calf at her side. A cow with a 365 day calving interval will never have her net energy requirements fall to maintenance alone because of the overlap from the net energy requirement for gestation and lactation. The weaning and dry-off period for a spring calving cow occurs in late summer/early fall. A beef cow will have her least nutrient requirements of the year when she ceases lactation, and her calf is weaned.
At the end of gestation, beef cows experience their second greatest requirements for energy. It is during this time that many producers underestimate the energy requirements of their cows. During the last 90 days of gestation, cows experience an exponential increase in net energy and protein requirements for fetal growth and must also prepare for lactation and parturition. These processes require large amounts of energy, and in spring calving herds this stage of production commonly coincides with poor forage quality and environmental stressors such as cold temperatures and more recently, muddy conditions. Research has been completed at The Ohio State University by our research group to determine the impacts of muddy conditions on the energy requirements of beef cows during late gestation. We observed that cows housed in muddy conditions for the last trimester of gestation had an estimated increase in energy requirements of 3.9 Mcal Net Energy/day, which is equivalent to approximately 40% of the daily energy requirements for maintenance of a mature 1200 pound cow. Using average net energy values for feedstuffs from the NRC (2016), this 3.9 Mcal net energy/day would be equivalent to supplementing cows with either approximately 2.5 lbs/day of whole shelled corn, 2.3 lbs/day of corn gluten meal, or 2 lbs/day of wet DDGS.
For spring calving cows, two areas for beef producers to focus on with respect to energy requirements of the cow are at the peak of lactation and in the last 90 days of gestation. The last 90 days of gestation aligns with late winter when cows do not have adequate forage to graze and are often provided hay that likely does not allow the cow to meet maintenance plus gestational energy requirements and the extra energy required to deal with cold stress and muddy conditions.
In our previous research, mature cows exposed to muddy conditions during the last trimester of gestation lost approximately 1 body condition score. Cows housed in mud began the trial at a BCS of 5 and ended at a BCS of 4 at calving. These mature cows were able to mobilize their own body tissues to meet the nutritional deficit and still provide for fetal growth as we did not observe any differences in calf birth weight. While a loss of 1 BCS during late gestation did not affect calf birth weight in our research, it is crucial to avoid weight losses during this stage of production that result in body condition scores of 4 or less at calving (BCS 1-9 scale). Cows with reduced body weight and body condition scores (< 4.5) can have decreased colostrum quality, longer postpartum intervals, increased days to conception, and decreased pregnancy rates (Corah et al., 1975; Soca et al., 2013; Selk et al., 1988; Perry et al., 1991). With currently rising feed and commodity costs, it is important to carefully consider the energy requirements of your cow herd and the different supplementation options if your cows are going to be housed in muddy conditions during the last trimester of gestation.
Source : osu.edu