Effects of supplementing beef cows grazing spring pasture

Many producers turn lactating beef cows out on pasture in early spring, just prior to breeding attempts. At this time, early forage growth tends to be characterized by a relatively high ratio of crude protein (CP) to total energy, and dry matter (DM) is at its lowest level (Kirchhof et al., 2010). As the season progresses and the plant matures, CP as a percent of DM declines, while total percent DM increases (King et al., 2012). Indigestible fiber, in contrast, is fairly low in early spring, increasing throughout the season (King et al., 2012). Acid detergent fiber (ADF) and neutral detergent fiber (NDF) are measures of indigestible fiber. Because spring pasture is so moist, gut fill can become an issue; if the cow is unable to consume more energy than she expends, she experiences a negative energy balance, possibly lowering weight and body condition. This can also be due to excess protein in the diet, as may be the case in early forage growth (Gross et al., 2011). While in a negative energy balance, the cow may have to mobilize body reserves to generate metabolites that can be used for energy (Gross et al., 2011). These alternative fuels are often the result of catabolism, usually reducing weight and body condition (Cincović et al., 2012). To generate these energy compounds, the cow’s body may initiate lipolysis and ketogenesis (Cincović et al., 2012). Due to breakdown of fat and suppression of lipid production, concentrations of non-esterified fatty acids (NEFA) in the blood rise (Cincović et al., 2012). NEFA can be utilized for energy or oxidized to ketones, like beta-hydroxybutyrate (BHBA), in the liver (Cincović et al., 2012). Blood urea nitrogen (BUN) is another indicator of negative energy balance, as it can denote insufficient energy or exces