In two experiments with cows in early lactation fed 40% corn silage and 60% concentrate diets (dry basis), we examined effects of 1.0% dietary sodium bicarbonate and .8% magnesium oxide in a 2 x 2 factorial arrangement on performance, acid-base metabolism, and digestion. In Experiment 1 with 20 Holstein and 4 Jersey cows, intake and milk production were not affected by treatment through 12 wk postpartum. In digestion trials at 3, 6, 9, and 12 wk postpartum, addition of magnesium oxide increased digestibility of dry matter 1.8% units by sodium bicarbonate whereas acid detergent fiber digestion increased 9 to 12% units with addition of either buffer. In Experiment 2, with four fistulated Holstein cows in a Latin square design, intake per unit body weight was increased .18% units by dietary sodium bicarbonate while milk production was unaffected. Dry matter and acid detergent fiber digestion were increased slightly by sodium bicarbonate plus magnesium oxide addition whereas nitrogen balance increased 23 g/day in cows fed magnesium oxide alone. Because of the low fiber content of the ration, milk fat percent increased .5 to .9% by addition of either buffer in both experiments and resulted in corresponding increased in fat-corrected milk. Sodium bicarbonate and magnesium oxide prevented rapid declines in rumen pH associated with feeding in Experiment 2, which related to changes in acid detergent fiber digestion. Blood acid-base metabolites were relatively unchanged by treatment in either experiment. Urinary bicarbonate and net acid excretion rates were changed on the average by +41 and +43 meq/h by sodium bicarbonate or magnesium oxide but could not be related to changes in intake and production.

Two experimental strains of tall fescue grass (Festuca arundinacea Schreb) that had previously been shown to produce differences in animal performance during the summer period were fed to Holstein calves in temperature controlled rooms. In the first year, GI-306 (less toxic) and GI-307 (more toxic) tall fescues were fed during July in rooms maintained at 10 to 13, 21 to 23 and 34 to 35 C. In the second year, the same tall fescues and orchardgrass were fed at a high temperature during May (32 to 33 C), July (34 to 35 C) and October (31 to 32 C). A comparison at a lower temperature (16 to 18 C) was also included in the July and October trials, On both years, calves consuming GI-307 tall fescue consistently had lower dry matter intakes, higher rectal temperatures, higher respiration rates and lower weight gains than those fed GI-306 tall fescue or orchardgrass at temperatures above 31 C. At lower ambient temperatures, differences in dry matter intake, rectal temperature and respiration rates between animals on the two strains of forage were not statistically different. The results demonstrate the poor performance of cattle grazing fescue is related to environmental temperature and that the toxic substance(s) is present throughout the growing season.

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Twenty cows were in an experiment to measure effects of dietary buffers, sodium bicarbonate and magnesium oxide, on ration adjustment and incidence of metabolic problems in the first 8 wk postpartum. Cows were fed 2.7 kg grain per day and alfalfa ad libitum prepartum and switched immediately to a complete ration of 40% corn silage and 60% concentrate (dry matter) postpartum. Treatments included 1.5% sodium bicarbonate and .8% magnesium oxide (total ration dry matter) fed in a 2 x 2 factorial arrangement. Cows receiving bicarbonate peaked 2 to 3 wk earlier in intake and averaged 2.1 kg per day greater intake than those fed the control diet. Actual milk production was increased by sodium bicarbonate with greatest differences for cows receiving both buffers averaging 3.8 kg per day more milk than control cows. Increases over co ntrols were 2.6 and 5.6 kg fat-corrected milk per day for cows receiving sodium bicarbonate alone or in combination with magnesium oxide, respectively. Sodium bicarbonate increased acetate to propionate molar ratios in rumen samples taken at 1 and 2 wk postpartum whereas neither buffer had any effect on rumen pH. Blood hematocrit and urine pH were not affected by treament. Magnesium oxide increased fecal pH .8 units and slightly decreased fecal starch, but sodium bicarbonate had no effect.