Source: South Dakota State University Extension
The need to feed hay is not eliminated through winter grazing, especially during times of inclement weather and as spring cows advance in pregnancy. However, winter grazing can be used to reduce the amount of supplementation required via hay or other feeds. After weaning, the average nutrient requirements of a cow are about 50% total digestible nutrients (abbreviated as TDN) and 7% crude protein (abbreviated as CP). Typically, at the start of grazing, dormant grasses contain less than 6% CP. Protein is generally the most-limiting nutrient during winter grazing, but energy is also important for combating winter conditions and increasing body condition score. A spring cow’s nutrient requirements will increase throughout the winter as they advance into late gestation. As a result, cows should be provided with adequate protein, energy, and mineral supplementation throughout the winter grazing season in order to meet their nutritional requirements.
Assessing cow nutrient requirements.
In order to provide adequate nutrition to the cow herd, it is essential to assess nutrient requirements. An easy way to accomplish this is to evaluate the herd for body condition scores (abbreviated as BCS). For most spring calving herds, fall and winter grazing occurs after the calf has been weaned. As a result, cows will be at their lowest nutritional requirements of the production cycle (Figure 1). Therefore, this period between weaning and late gestation is the most-economical time to increase cow conditions if needed. Current BCS of the herd will determine what feeding, supplementing, and management strategies should occur. Ideally cows would be in a BCS of 5 at the time of calving. However, nutrient requirements for heifers and younger cows are higher than for mature cows because they are still growing. As a result, younger cows may require more supplementation and ideally should be in a BCS of 6 at the time of calving.
One strategy is separating cows into groups based on their BCS. By managing the groups separately, supplementation can be provided according to the nutrient needs of each group. For example, one group could be made up of thinner cows (< 4 BCS) and young cows in order to provide more supplementation to achieve a higher energy and protein diet. In contrast, cows in a higher body condition (> 6 BCS) can be grouped together because they have more body reserves and are more resilient to low-quality diets. This group will require less supplementation and will be able to occasionally ‘rough it’ if necessary. However, caution should be used with this strategy by not allowing cows to lose too much condition prior to the calving season. Ultimately, winter grazing management should revolve around utilizing winter pastures as a feed resource, while ensuring cows will be in proper condition at the time of calving.
Does winter grazing impact cold stress in cows?
When temperatures are below their lower critical temperature, cows are forced to increase heat production to maintain their body temperature. In order to produce more heat, cows are forced to mobilize energy via body stores or diet intake. Thin cows are less-resilient to cold stress conditions, because they have less body stores available to produce heat and provide insulation from the cold. In order for thinner cows to not pull from body stores and lose condition, energy intake must increase.
The only way to get more energy from dormant forage is for cows to eat more of it. However, cow diets are limited by their rumen capacity. When forage quality is low, it is likely that the cow won’t be able to consume enough forage to get the energy they need. In this case, they are physically limited by intake. As a consequence, cows will have to mobilize energy from their body stores and risk losing body condition. To prevent losses in cow performance and condition, producers should provide supplemental energy to cows prior to and during cold stress events. By doing this, cows are able to increase both their energy intake and digestion of low-quality forage.
However, too much energy can cause digestive issues. For this reason, supplemental energy in the form of high-quality forage (for example, alfalfa) is a good strategy for shorter-term cold stress events. In the event of long-term cold events or extreme weather conditions, further management considerations may be required, and diet density should increase. Contact SDSU Beef Specialists to further discuss diet considerations.
Will cow nutrient requirements change throughout the winter?
Cow nutrient requirements change throughout the year as they enter different stages of gestation and lactation. The greatest nutrient requirements occur after calving and during peak lactation, while the lowest nutrient requirements occur after weaning, when lactation is stopped (Figure 1).
For spring calving cows, weaning typically occurs near the time that grasses go dormant for the winter. As a result, it is advantageous to utilize dormant grasses for spring calving cows during this period of lower nutrient requirements. However, as cows advance through pregnancy and get ready to calve again, their nutrient requirements are steadily increasing. Therefore, it is a good strategy to utilize winter grazing early on when nutrient requirements are lower.
Regardless of grazing strategy, it is critical that producers provide proper supplementation to allow cows to meet nutrient requirements throughout the winter grazing season.