For many cattle producers, it is a relief to put the cows to pasture. We have just overcome winter feeding obstacles, and many of us are ready to turn the cows out and focus on other tasks. However, are you certain they are receiving the nutrients required to support their calf?
Lactation is a demanding physiological state and a crucial time to meet nutrient requirements to support production performance. First, let us look at the macronutrient requirements of a 1,200-pound lactating beef cow. She is going to require 9.8% crude protein (CP) and 58% total digestible nutrients (TDN). These specific values may differ by weight and other livestock species such as dairy cattle or sheep and goats.
It will be important to ensure she is meeting her requirements for milk production and to maintain her own body condition. If she loses too much body condition supporting a calf on poor-quality pastures, it will negatively impact her ability to breed back for the next calf crop. Furthermore, if she is losing body condition, she is more likely to stop lactating sooner, thereby contributing to low weaning weights. In addition to low weaning weights, poor immunity and post-weaning performance of the calf may result from poor lactation performance. All these factors have a negative economic impact on livestock operations. So it is key to meet nutrient requirements during this phase of production.
The risk of high fiber
Now, let us think about the pasture in spring months. The forages available in the early spring months are going to be growing rapidly. Available biomass will be minimal and then plentiful. This is good because there is forage there for gut fill when the beef cow is willing to consume the most. However, rapid forage growth can result in high-fiber forages. High-fiber forages are lower in energy and protein. Therefore, the nutrition you expect on the lush green pastures may not actually be available to your herd. This variability in pasture forages means animal nutrient requirements which may or may not be met.
Figure 1 shows pasture samples submitted for laboratory analysis in the months of March, April and May.
These samples were submitted from Nebraska, Wyoming, Kansas and South Dakota. While they may not represent your region, they draw attention to the importance of precise animal nutrition in all seasons. CP, TDN, acid detergent fiber, amylase-treated neutral detergent fiber and neutral detergent fiber digestibility at 48 hours (NDFD) were all plotted as box and whisker plots. The box represents the range which contains 50% of the laboratory data, while the whiskers indicate the range of the data or, in this case, the variability of spring pasture forages.
The first, examining CP, the beef cow’s requirement falls within the data spread. This means cows on pasture may or may not be meeting protein requirements. It is not a guarantee that pasture forages contain 9.8% CP. When adjusting these values to compare your animals’ needs, consider the constant line or requirements moving. For example, a 175-pound ewe supporting twin lambs will require 14.8% CP. The constant line representing her requirements would move up, meaning fewer samples are at or above her requirement. Therefore, pasture forages are less likely to be meeting her protein requirements.
Next, looking at TDN, we see a similar trend with samples above and below the beef cow requirement of 58%. Again, indicating energy supplementation may or may not be required. Often, TDN is calculated based on the ADF value. Figure 1 shows the samples ADF value plotted; it is a mirror image of the TDN to reinforce the concept that fibers are inversely related to the energy content of the forage.
Fiber on pasture
Continuing with an examination of the fibrous content of forages, aNDF and NDFD were plotted. As aNDF increases, rumen passage rate is impeded. To pass from the rumen to the omasum, the particle size of the forage must be 1 millimeter. Amylase-treated NDF is the slowly digestible portion of the feed and is the last to meet this size reduction. Therefore, a higher concentration of aNDF means more time spent ruminating and less time grazing. Less time grazing means less dry matter intake (DMI).
This is important because in some forages, protein and TDN can meet an animal’s nutrient requirement as a percentage of the dry matter – but not in pounds per day due to impeded intake by high aNDF. However, aNDF should be examined in combination with digestibility. Higher NDFD means increased passage rate and increased intake. NDFD has been correlated with higher milk production. Samples plotted in Figure 1 once again fall into a range which shows variation on pasture.
Next, let us turn our attention to the micronutrient requirements of our calving and lactating cattle on spring pastures. As a beef cow moves from gestation to lactation, her magnesium requirements increase from 0.12% to 0.2% of total DMI. Calcium is another key nutrient when shifting from gestation to lactation. A beef cow’s calcium requirement increases from 0.18% to 0.31% of total DMI. Accompanying calcium metabolism, phosphorus is required at 0.2% of total DMI. Producers should be conscious of keeping these minerals in balance through proper supplementation. Other micronutrients of concern on spring pasture are zinc and copper. These minerals play an integral role in immunity and are commonly deficient in most forages relative to beef cattle nutrient requirements.
Avoiding grass tetany
Many producers are aware that grass tetany is a risk on lush spring pastures. Rapid-growing, cool-season grasses in spring pastures are commonly deficient in magnesium. However, magnesium deficiency is not the only mineral playing a role in grass tetany. High-potassium forages also cause grass tetany. Potassium interferes with magnesium absorption, thereby contributing to its deficiency. Forages with more than 3% potassium on a dry basis should be supplemented with a high-magnesium mineral supplement (8% to 12% Mg). It would be advantageous to producers to sample pasture forages and choose a mineral supplement based on reported results.
In conclusion, we are not focused on nutrition in the spring. Producers test their hay in the fall and meticulously prepare to feed animals through the winter. However, conquering winter feeding challenges does not mean we should lose focus. Spring calving is a crucial production phase. Precise knowledge of available nutrients on pasture will allow producers to provide precise supplementation programs. A well-managed supplementation program will ensure optimal production performance and preclude wasted feed supplement costs. Pasture forage analysis is an economical tool for livestock producers.
Originally Printed for Progressive Forage February 2021.