Reactive oxygen metabolites generated during normal metabolism and metabolism stimulated by xenobiotics can enter into reactions that, when uncontrolled, can impair performance of dairy cows. Direct effects include peroxidative changes in membranes and other cellular components. Indirectly, competitive consumption of reducing equivalents can interfere with important metabolic functions and divert glucose from other pathways by inducing the monophosphate shunt. Normally, the body is protected by a wide range of antioxidant systems working in concert. Metal catalysts of oxidative reactions are removed in intracellular fluids by metal-binding macromolecules. Superoxide dismutases, glutathione peroxidase, and catalase within cells remove superoxide and peroxides before they react with metal catalysis to form more reactive species. Finally, peroxidative chain reactions initiated by reactive species that escaped enzymatic degradation are terminated by chain-breaking antioxidants, including water-soluble ascorbate, glutathione, and urate and lipid-soluble vitamin E, ubiquinone, and beta-carotene. To optimize performance, oxidative stress in high producing cows must be controlled by supplying all known antioxidant nutrients and by minimizing effects of substances that stimulate reactive oxygen metabolites.

In this study, we present evidence that cows fed highly cationic diets are less responsive to parathyroid hormone than those fed a highly anionic diet. Forty-seven Jersey cows (55 mo of age) were fed an alfalfa haylage-based diet supplemented with either anions (Cl-) or cations (Na+). Cows fed the high cationic diet suffered significantly more cases of milk fever (6 out of 23) than those fed the high anionic diet (1 out of 24). Concentrations of Ca at parturition and the first 2 d of lactation were significantly higher in cows fed the anionic diet. Secretion of parathyroid hormone in response to developing hypocalcemia was similar in cows fed either diet. Plasma hydroxyproline concentration (an index of bone Ca resorption activity) was greater in cows fed the anionic diet, suggesting better utilization of bone Ca. Plasma 1,25-dihydroxyvitamin D concentration was correlated inversely with plasma Ca and related directly to plasma parathyroid hormone in both groups of cows. However, the magnitude of the response (the amount of 1,25-dihydroxyvitamin D produced per unit increase in parathyroid hormone) was reduced greatly in cows fed the high cation diet. Because parathyroid hormone regulates both bone Ca resorption and renal 1,25-dihydroxyvitamin D production, these data suggest that prepartal diets high in cations decrease the ability of bone and renal tissues to respond to parathyroid hormone. Addition of anions to prepartal diets can reduce the excess cation balance of diets, increasing tissue response to parathyroid hormone and enabling the cow to better adapt to the Ca demands of lactation.

Jersey cows were fed three alfalfa haylage-based diets with different cation-anion balances beginning 6 wk preceding third or later calving and ending 24 to 36 h postpartum. Sodium and Cl as percentages of dietary DM were .08 and 1.66 in diet 1 (anionic, 5 cows), .44 and .91 in diet 2 (intermediate, 6 cows), and 1.60 and .34 in diet 3 (cationic, 6 cows). Cation-anion balances were 22, 60, and 126 meq/100 g DM; Ca:P ratios averaged 4:1. Cows fed diet 1 in comparison with cows fed diets 2 or 3 over 6 wk had similar concentrations of Ca, P, and Na but higher concentrations of Mg and K in plasma and higher urinary excretions of Ca and Mg. Concentrations of 1,25-dihydroxyvitamin D 3 d before parturition were higher in cows fed diet 1 than in cows fed diets 2 or 3. Within 36 h after calving, mean concentrations of Ca in plasma (mg/dl, range) of cows fed diets 1 to 3, respectively, were 7 (8.7 to 6.2), 6.5 (7.8 to 3.9), and 6.3 (7.8 to 3.8). Number of cases of clinical milk fever by diet were 0 of 5, 2 of 6, and 1 of 6 cows. Alteration of dietary cation-anion balance by addition of Cl may effectively reduce incidence and severity of parturient hypocalcemia.

Relationships between residual feed intake (RFI) and other performance variables were determined using 54 purebred Angus steers. Individual feed intake and BW gain were recorded during a 70-d post-weaning period to calculate RFI. After the 70-d post-weaning test, steers were fed a finishing ration to a similar fat thickness (FT), transported to a commercial facility, and slaughtered. A subsample of carcasses (n = 32) was selected to examine the relationships among RFI, meat quality, and palatability. Steers were categorized into high (> 0.5 SD above the mean; n = 16), medium (mid; +/- 0.5 SD from the mean; n = 21), and low (< 0.5 SD below the mean; n = 17) RFI groups. No differences were detected in ADG, initial BW, and d 71 BW among the high, mid, and low RFI steers. Steers from the high RFI group had a greater DMI (P = 0.004) and feed conversion ratio (FCR; DMI:ADG; P = 0.002) compared with the low RFI steers. Residual feed intake was positively correlated with DMI (r = 0.54; P = 0.003) and FCR (r = 0.42; P = 0.002), but not with initial BW, d 71 BW, d 71 ultrasound FT, initial ultrasound LM area, d 71 ultrasound LM area, or ADG. The FCR was positively correlated with initial BW (r = 0.46; P = 0.0005), d 71 BW (r = 0.34; P = 0.01), and DMI (r = 0.40; P = 0.003) and was negatively correlated with ADG (r = -0.65; P = 0.001). There were no differences among RFI groups for HCW, LM area, FT, KPH, USDA yield grade, marbling score, or quality grade. Reflectance color b* scores of steaks from high RFI steers were greater (P = 0.02) than those from low RFI steers. There was no difference between high and low RFI groups for LM calpastatin activity. Warner-Bratzler shear force and sensory panel tenderness and flavor scores of steaks were similar across RFI groups. Steaks from high RFI steers had lower (P = 0.04) off-flavor scores than those from low RFI steers. Cook loss percentages were greater (P = 0.005) for steaks from low RFI steers than for those from mid RFI steers. These data support current views that RFI is independent of ADG, but is correlated with DMI and FCR. Importantly, the data also support the hypothesis that there is no relationship between RFI and beef quality in purebred Angus steers.

Possible relationships among dietary antioxidants, oxidative status, and placental retention were investigated in periparturient dairy cows. During 6 wk prepartum, 16 cows each were given daily by capsule 1000 IU of vitamin E, 3 mg of Se, both vitamin E and Se, or neither (control). alpha-Tocopherol in serum and fast-acting antioxidants in plasma increased, but, in red blood cells, thiobarbituric acid-reactive substances decreased during the last 6 wk before parturition in cows given vitamin E. These measurements were unaffected by supplementation of Se. Cows that had retained placenta > or = 12 h had lower fast-acting antioxidants in plasma and glutathione peroxidase in red blood cells up to 2 wk before calving than did cows that shed fetal membranes in < 12 h. Results suggest that inadequate dietary antioxidants may increase oxidative stress, production of lipid peroxides, and incidence of retained fetal membranes in dairy cows.