Introduction

Biological Criteria for Selecting Fat Supplements

To maximize feed intake, ruminal fiber digestion, milk fat percentage, and animal health, it is essential to feed fats which minimize interference with ruminal fermentation. Elevated concentrations of unsaturated nonesterified fatty acids in the rumen should be avoided (9). Unsaturated free fatty acids reduce microbial digestion of fiber. Most dietary fat is in triglyceride form; three fatty acids esterified to glycerol. Feeding triglyceride does not avoid the presence of free fatty acids in the rumen because triglycerides may be hydrolyzed by ruminal bacteria. Once hydrolyzed, bacteria can hydrogenate fatty acids. However, if large amounts of unsaturated fatty acids are present, the capacity for biohydrogenation is exceeded. Concentration of unsaturated fatty acids in the rumen is largely dependent on the amount and type of fat fed, but is also influenced by the rates of triglyceride hydrolysis, fatty acid hydrogenation, fatty acid incorporation into microbial lipid, formation of calcium salts, and adherence to feed particles (9).

Oilseeds, relatively saturated fats, and mineral salts of fatty acids are desirable fats for dairy rations because they have minimal effect on rumen fermentation. Oilseeds may contain 15 to 35% fat, predominantly in the triglyceride form. Oilseed fat is usually highly unsaturated, which may seem undesirable. However, the fat within the seed is slowly released into the rumen as the seed is being degraded by ruminal microorganisms. Therefore, at any point in time, only small amounts of the fat are present in "free" form in the rumen. Feeding a totally mixed ration or heat treated oilseeds may further minimize the amount of fat released into the rumen at any particular moment. The negative impact of unsaturated fat is reduced under these conditions, largely because the bacteria in the rumen are able to convert modest amounts of unsaturated fat to less toxic saturated fat.

Saturated fats are believed to be relatively inert in the rumen because of their high melting point and consequently low solubility in rumen fluid (2). There is limited evidence to suggest that triglycerides are less detrimental to rumen fermentation than fatty acids (2). Rate of triglyceride hydrolysis may be slower for saturated fats (14). Reducing solubility of fatty acids in rumen fluid may reduce interaction with rumen microorganisms. The fatty acid composition and ratio of unsaturated fatty acids to saturated fatty acids of several fat sources is shown in Table 1. Tallow is solid at room temperature and is considerably more saturated than vegetable oils and greases.

Mineral salts of long chain fatty acids are inert in the rumen because they are relatively insoluble in ruminal fluid (2). Once the complex is delivered to the acidic conditions of the abomasum and small intestine, the complex is broken and the fatty acid is available for digestion. The product has been extensively researched and is inert in the rumen when included at 4% of ration DM (16).

In addition to the effects of fats on rumen fermentation, digestibility of fats must also be considered when selecting a fat source. The single most important factor in determining the energy value of a fat is the amount absorbed by the intestine versus excreted in the manure. In general, characteristics of a fat that cause rumen inertness do not favor intestinal digestion (7). An exception is salt formation which promotes rumen inertness without adverse effects on digestion by the small intestine. Fat sources that are highly saturated are ruminally inert, but usually poorly digested (5). Decreasing particle size, feeding in the nonesterified form, or increasing the ratio of C16:C18:0 may enhance digestibility, but the influence of these factors on digestion is relatively subtle in comparison to degree of saturation. Fatty acids that are fed in esterified form should have an IV above 35 to maximize digestion in the small intestine. Decreasing IV below 35 has greater detrimental effects on digestion then beneficial effects on rumen fermentation. In general, fats with an IV between 35 and 50 can be included up to 3% of diet DM without detrimental effects on rumen fermentation. Feeding above 3% of diet DM will probably have greater negative effects on digestibility than on rumen fermentation. Numerous studies have indicated that cattle have limited capacity to digest fat; feeding more than 1000 g fat per day may result in reduced digestibility in the intestine.

Cost of Fat Supplements

Developing Feeding Strategies For Supplemental Fat

Many feel there should be an upper limit on the amount of fat provided to the diet by oilseeds. The concern usually is polyunsaturated fatty acids within oilseeds interfering with fiber digestion in the rumen. Feeding free vegetable oil is likely to reduce fiber digestion and/or milk fat percentage. However, detrimental effects are much less likely if the oil is encapsulated in a seed and introduced into the rumen in small doses over time by feeding a totally mixed ration (12). Relatively large quantities of oilseeds can be fed in a total mixed ration with beneficial results (11, 17). While it may be possible to overfeed oil as part of whole oilseeds, some other nutritional factor will probably limit the feeding of oilseeds. The amount of raw oilseeds fed may be limited so that the diet does not contain too much degradable protein or trypsin inhibitor (i.e. soybeans; 1, 21). Cost of overfeeding undegradable intake protein may limit the amount of heated oilseeds that are fed. The amount of cottonseed fed may be limited to 3.6 kg/day whole cottonseed or 4.5 kg/day of whole cottonseeds plus cottonseed meal to minimize gossypol toxicity. Feeding extruded oilseeds represents an exception in which oil induced milk fat depression may occur even if the amount fed is restricted to meet protein requirements of the cow. During extrusion, oil is released from the seed and behaves as free oil. A summary of 19 trials (20) indicated that milk fat was .17 percentage units lower when feeding extruded soybeans as compared to feeding soybean meal or raw soybeans.

Once oilseeds are incorporated into the diet to balance the protein and/or fiber needs of the cow, one must decide if there should be additional fat in the diet. A summary of research trials in which a second increment of supplemental fat was added to diets already containing supplemental fat is in Table 3. Most studies have insufficient animal replication to identify treatment differences for milk yield, therefore, data from numerous studies have been composited. There are problems with this approach because conditions in which trials were conducted are not consistent. Studies were divided into those in which the second source of fat was granular fat or liquid fat. Average amount of supplemental fat prior to addition of the second source of fat was 2.3% for trials in which granular fats were fed and 2.2% for trials in which liquid fats were fed. Average level of supplementation for the second source was 2.2% for granular fats and 2.6% for liquid fats. Average milk yield of control cows (single fat source) was 33.9 and 34.8 kg/day for comparisons involving granular and liquid fats, respectively. Daily dry matter intake (DMI) was reduced 0.36 kg when feeding granular fats and 0.04 kg when feeding liquid fats. Milk yield response was relatively similar when feeding granular versus liquid fats (0.77 kg/day versus 0.63 kg/day), but feeding granular fats resulted in approximately 0.95 kg/day FCM response advantage over feeding liquid fats (1.08 versus 0.13 kg/day). This was the result of granular fats causing an increase in milk fat percentage (+.08) and liquid fats causing a decrease in milk fat percentage (-.1). Feeding total supplemental fat at 6% or more of ration DM caused reductions in DMI and/or milk yield (4, 15, 23). Very few studies have been conducted to compare various fat sources as a second increment of fat in dairy diets. Wu et al. (22), added 2.5% tallow, calcium salts of long chain fatty acids, or prilled hydrogenated tallow fatty acids to diets containing 2.4% whole cottonseed. Fat supplementation resulted in greater milk and FCM yield, and decreased milk protein; however, there were no significant differences between fat sources.

Results in Table 3 indicate that use of a second source of fat cannot be justified economically on the basis of milk response alone. Milk yield responses diminish as the amount of supplemental fat increases (13). However, to obtain optimum health and reproductive performance of dairy cattle, one must avoid extreme losses in body condition. Lost body condition should be replenished by the end of lactation to increase the likelihood of optimal performance during the following lactation. If cows are routinely loosing more than 1.0 in body condition score (1=thin to 5=obese) and/or are not reaching a body condition score of 3.5 by the time they are dried off, higher energy diets may be justified. Cereal grains are usually cheaper per unit of energy than fat. Replenishment of body condition should be obtained by feeding more grain if fiber levels do not fall below recommended levels (28% NDF, DM basis). If grain feeding is already maximized, additional fat supplementation may be warranted.

Results in Table 3 also indicate that it may be difficult to justify the extra cost of granular fats compared to liquid fats based on superior animal performance. Although tallow and grease are not as inert in the rumen as granular fats, they are relatively saturated in comparison to soybean oil which has an unsaturated:saturated ratio of 5.6 (Table 1). Tallow is the most saturated with a unsaturated:saturated fatty acid ratio of .96 while yellow grease has a ratio 2.44.

California research (3) indicates moderate amounts of yellow grease can be added to diets containing whole cottonseed without interfering with rumen fermentation. Tallow was added at 1, 2, or 3% of diet DM to a totally mixed diet already containing 2.8% supplemental fat from whole soybeans (8). Cows were at peak intake so cows receiving the highest amount of tallow were consuming 1.8 kg of supplemental fat per day. There were no adverse effects of tallow on feed intake, rumen fiber digestion, and milk fat percentage.

These studies suggest that tallow and greases were essentially inert in the rumen, therefore, moderate quantities can be fed in addition to oilseeds when cows are fed a totally mixed ration. A note of caution: diets fed in these studies were alfalfa based. Evidence indicates that fat may be more likely to interfere with rumen fermentation when the forage source is corn silage. Feeding 12% whole cottonseed depressed milk yield, fat percentage, and DMI when 100% of the forage was corn silage, but not when alfalfa silage or a 50:50 alfalfa-corn silage diet was fed (19). In the same study, tallow supplementation did not alter milk fat percentage when corn silage was fed and increased milk fat percentage when a portion of the corn silage was replaced with alfalfa hay. Tallow tended to increase milk production more when cows were fed all corn silage diets versus corn silage/alfalfa hay diets. Examination of nutrient digestibility data led the authors to conclude that fats may have a more negative effect on fermentation when diets are based on corn silage as the only forage. Smith and Harris (18) compiled data from lactation trials in which various fat sources were fed with either corn silage, alfalfa, or corn silage/alfalfa-based diets. They concluded that there was a greater likelihood of decreased milk and/or milk fat percentage when extruded soybeans, whole cottonseeds, and rendered animal fats were fed with corn silage-based diets as compared to alfalfa-based diets. Response to granular fats that are designed to be ruminally inert were not affected or inconsistently affected by forage type. Mechanisms by which fat may be more detrimental to fermentation on corn silage based diets have not been identified; rate of fiber disappearance from the rumen may be important.

Granular fats are easy to handle relative to liquid fats and may be preferred for that reason. Granular fats may also be warranted when supplemental fat can not be slowly introduced into the rumen, e.g., when frequent concentrate feeding or a totally mixed ration is not feasible. Granular fats are more inert in the rumen than other fat sources, therefore, they should be considered when less inert fats are suspected of causing depressed DMI, ruminal fermentation and/or milk fat percentage. This may be more likely when feeding high grain diets (6) or diets that are not alfalfa based (18).

Conclusions and Recommendations

Additional Readings

Alberta Dairy Management Fact Sheet:
Facts About Fats

References