Feed Additives for the Dairy Herd: A Literature Review and Discussion of Practical Applications JACK RODEN BURG I N T R O D U C T I O N Since the advent of artificial insemina- tion and modern genetic evaluation techniques, rapid genetic improve- ment for milk production has gener- ated a number of challenges for the dairymen and dairy nutritionists. Higher milk production in early lacta- tion, coupled with a limited capacity to consume feed results in weight loss during the first ten to 12 weeks post- partum, which can limit productivity and lead to reduced reproductive per- formance and health problems. Higher rates of grain feeding and the use of high energy, low fibre, often ensiled forages is intended to minimize this "energy crisis", but can alter rumen fermentation leading to fat test depression. Researchers have examined a number of traditionally unsupple- mented, and nonnutritive compounds which show potential to correct these problems and to increase productivity. These compounds, loosely referred to as "feed additives" are generally well received by dairymen, seeking instant solutions and some do become incor- porated into generally accepted feed- ing practices. The following feed addi- tives are reviewed with emphasis on practical feeding guidelines and dis- cussion of economic impact of their use. Sodium Bicarbonate This rumen buffer is a major con- stituent, 0.62% of saliva and performs a significant role in maintaining near neutral pH in the rumen despite pro- duction of volatile fatty acids. When diets low in fibre cause a reduction in cud chewing and natural production of sodium bicarbonate in saliva, rumen pH decreases causing a shift in the microbial population and a decrease in acetic acid and increase in propionic acid. This leads to hormonal changes which divert energy away from the production of milk fat, there- by causing a fat test depression. Research has shown that the addition of sodium bicarbonate can at least par- tially compensate for the reduced sal- iva production and increase fat test in cows on low fibre diets (1). In view of its mode of action, when rumen func- tion is normal, the addition of sodium bicarbonate should not be expected to raise fat test above the genetic poten- tial of the cow. In milk recorded herds, it is useful to examine fat and protein tests of early lactation cows to assess whether or not the fat test is "normal". Because fat and protein percent have a strong positive genetic correlation, cows with a low fat test attributable to genetics or stage of lactation should also have a low protein test. In Hol- steins, fat test is typically 0.4 to 0.5% higher than protein. If a large percent- age of cows have fat tests below this level, ration changes to improve the level of effective fibre, or the addition of sodium bicarbonate is justified (2). Higher levels of sodium bicarbonate in the rumen, whether naturally pro- duced or from a feed additive, also increase fibre digestion and rate of feed passage through the rumen (3,4,5). This side effect may be respon- sible for the increased appetites and reduced incidence of displaced abom- asum, laminitis, ketosis and other health problems reported in many research trials (6,7). The addition of sodium bicarbonate at a rate of 200 grams per cow per day or 1.5% of the grain ration is generally recommended in fat test depression situations. At a cost of approximately I I c per cow per day, it should be noted that dairymen could alternatively spend a similar amount to feed an additional 1.5 kg of hay which sup- plies other nutrients in addition to promoting normal fat test. When a whole herd is suffering from fat test depression, changing the forage pro- gram may be the most economical solution, however even with good roughage programs, early lactation cows producing over 35 kilograms of milk per day are consuming a diet of close to 60% grain on a dry matter basis. For these cows, the routine addi- tion of the above level of sodium bicarbonate is probably justified. Sodium bicarbonate should be intro- duced gradually to avoid scouring and feed refusal (8). Since this additive also supplies sodium, salt feeding may be reduced by I gram for each 1.4 grams of bicarbonate fed to a minimum of half normal salt levels. Cows fed sodium bicarbonate commonly ex- hibit slightly lower levels of blood magnesium (9), therefore rations should be analysed to ensure adequate magnesium is supplied. Magnesium Oxide This mineral compound is also commonly classified as a buffer, although its ability to stabilize rumen pH is limited and dependent on parti- cle size (10,11). Magnesium also plays a major role in the synthesis of milk fat in the udder (12,13) and is required in the diet at a level of at least 0.23% of dry matter intake (14). In spite of this, research trials reporting a fat test response to added magnesium oxide, seldom report the total magnesium level in the diet. Levels up to 100 grams per cow per day or 0.8% of grain mix are commonly recommended to cor- rect fat test depression (15%). Since this level may reduce ration palatabil- ity and dry matter intake, it may be P.O. Box 666, Woodstock, Ontario N4S 7Z5. 40