Changes milk fat lactating cow

The fat percentage of milk or butterfat has a significant economic impact for dairy producers. In recent years, the fat increment in milk has varied from less than 7 cents per tenth of a percent butterfat above or below 3.5 percent fat in the milk to over 20 cents per tenth of a percent of fat. Recently, the fat content of milk accounted for approximately 50 percent of the total value of the milk. Although fat rarely accounts for 50 percent of the value of the milk, small variations in fat percentage can significantly affect economic returns to dairy producers.

Milk fat depression can be a problem for the entire herd or for an individual cow. This fact sheet primarily addresses the problem in herds, but individual cows may selectively consume a ration that causes a depression in the fat percentage in their milk. Moreover, genetics of individual cows may translate into a low fat percentage in their milk.

Milk fat depression in a herd is usually defined as below 3.2 percent fat in milk for a Holstein herd or below 4.2 percent fat in the milk for a Jersey herd. Other breeds would have a problem with fat percentage when the fat percentage in milk is below 3.2 percent for Ayshires, 3.4 percent for Brown Swiss, 4.0 percent for Guernseys and 3.2 percent for Milking Shorthorn. However, marginal decreases of fat percentage above the levels indicated may be reversed by changes in management practices which are listed in this fact sheet and may be of economic importance.

For example, adding buffers to a ration or allowing cows to have continuous access to long-stemmed hay may result in an increase of 0.2 percent in the fat percentage in milk from 3.2 percent to 3.4 percent in a Holstein cow. However, the chances of this increase in fat occurring are very variable. In cases of severe milk fat depression, fat percentages may be 0.5 percent less than the values above. When a dairy herd has a problem with milk fat depression or the low fat syndrome, the ratio of milk fat:milk protein will be less than 1.0 for Holsteins. For example, milk fat may be 3.1 percent while milk protein may be 3.2 percent. In cases of severe milk fat depression, the milk fat:milk protein ratio will be less than 0.8.

Causes of Low Fat in Milk

The cause of low fat in milk can be several factors and is usually related to the feeding of the cow. One theory of milk fat depression relates to the production of trans-10, cis-12 conjugated linoleic acid in the rumen or other polyunsaturated fatty acids (PUFA) and their subsequent effect of slowing the synthesis of fat in the mammary gland. During milk fat depression, trans fatty acids are increased in levels present in the rumen and milk.

Other metabolic changes associated with low fat milk in the herd include: 1) a low acetate:propionate ratio in the rumen as acetate is one of the primary precursors of fat, 2) an acetate or B-hydroxybutyrate shortage which relates to acetate or B-hydroxybutyrate being necessary for the synthesis of fat within the mammary gland and 3) a low turnover of feed in the rumen where there are fewer nutrients coming from the rumen which again relates to less availability of nutrients available for syntheses of fat in the mammary gland.

Other theories, which have received less study, include 1) a deficiency of B-vitamins or magnesium which may be co-factors in the synthesis of milk fat as well as 2) the presence of other abnormal long-chain fatty acids which may tie up the mechanisms of fat synthesis. These different biological shifts within the cow’s rumen are related to lower rumen pH (usually below 6.0) associated with acidosis, which causes a shift in the microbial population within the rumen. However, most recent research suggests that milk fat depression is the result of changes in biohydrogeneration in the rumen, i.e., increased production of trans-linolenic acid, and is less likely due to changes in rumen patterns of volatile fatty acids (acetate, propionate, butyrate, B-hydroxybutyrate).

Management to Maintain Fat Values

If dairy producers want to obtain higher fat in the milk, the best economic alternative is including long-stemmed hay in the ration, usually at least 5 pounds per cow per day. It is very important to balance the ration of lactating cows to include 35 to 40 percent non-fiber carbohydrates, at least 20 percent acid detergent fiber, 28 percent neutral detergent fiber and 12 pounds of dry matter from forages, but these values may vary with the diet (Table 1). In most management programs, cows will need 15 to 18 pounds of dry matter from forages.

Table 1. Recommended minimum concentrations (% of DM) of total and forage NDF and recommended maximum concentrations (% of DM) of NFC for diets of lactating cows when the diet is fed as a total mixed ration with forage of adequate particle size and ground corn is the predominant starch source.¹

Minimum Forage NDF	Minimum Dietary NDF	Maximum Dietary NFC	Minimum Dietary ADF
19	25	44	17
18	27	42	18
17	29	40	19
16	31	38	20
15	33	36	21

¹ From “2001 Nutrient Requirements of Dairy Cattle”

Other good management practices that tend to maintain a stable milk fat percentage in dairy herds include regular feeding of clean, dry forage and grain to the lactating cows. This ensures that the cows have a stable diet and do not go on and off feed. For most dairy producers, it also is economical to include buffers such as sodium bicarbonate and/or magnesium oxide in the grain mix (Table 2) when more than 12 pounds of grain is fed per feeding in the milking parlor. If only sodium bicarbonate or sodium sesquicarbonate is added to a total mixed ration, it should be included as 0.75 percent of the ration dry matter.

Dairy producers who are wanting to increase the fat percentage in their herd will often add some of the other high fat breeds such as Jerseys. Jerseys will usually average 4.4 to 4.9 percent fat while Holsteins will generally average 3.2 to 3.8 percent fat in the milk. It is possible to breed for high fat percentage by selecting sires through artificial insemination that sire heifer calves that will have slightly higher fat percentage in the milk they produce. Within the cow’s natural lactation curve, her fat percentage will usually be the lowest within 6 to 10 weeks after calving and the highest toward the end of lactation. The exception to this general rule of highest fat in late lactation is when cows calve with a high body condition score. These cows mobilize fat in early lactation as they mobilize energy to produce milk, which translates into greater fat in the milk.

Table 2. Most effective sodium bicarbonate and magnesium oxide concentrations in rations for lactating cows¹

	% of concentrate	Lbs per ton of concentrate	Lbs per cow per day
Sodium Bicarbonate²	1.0 - 1.5	20 - 30	0.3 - 0.45
Magnesium Oxide²	0.4 - 0.8	8 - 16	0.1 - 0.2

¹ Bicarbonate is fed at 0.75 percent of the total ration dry matter in a total mixed ration, bicarb is fed at a level of 0.5 percent of the total ration dry matter and magnesium oxide at a rate of 0.2 percent of the total ration dry matter in a total mixed ration.
² When using sodium bicarb and magnesium oxide together, a 2:1 to 3:1 ratio is usually best and lower levels are used, e.g., 20 pounds of sodium bicarb plus 8 pounds of magnesium oxide per ton.

Troubleshooting Low Milk Fat in the Herd

Inadequate levels of effective fiber in the ration are the most common causes of a low milk fat test in the dairy herd (Table 1). The low levels of effective fiber in the ration may result from 1) overfeeding of concentrate mix as cows are milked, 2) the lack of available forage as pastures decrease in quantity and/or quality to be eaten by the cow, 3) consumption of large amounts of lush, fast growing pasture from annuals such as wheat, ryegrass, sorghum or millet, 4) consumption of alfalfa hay above 200 relative feed value or 5) corn silage or grass haylage that is too finely chopped.

A common cause of low fat in milk includes feeding forages that have a high level of neutral detergent fiber and acid detergent fiber plus a very low digestibility. Often as a result of poor digestibility in the forage, cows are fed excessive levels of grain, which contain large amounts of soluble carbohydrates such as starch or sugar. These soluble carbohydrates also contribute to changes in the microbial population of the rumen that causes a decrease in fat test. Additionally, sludge feeding or feeding large amounts of concentrate mix as cows go through the milking parlor can cause low fat test in the milk, especially when the concentrate mix does not include buffers. With few exceptions, cows should not be fed more than 15 pounds of grain per feeding as they go through the milking parlor.

Another factor that may contribute to a low fat test is allowing cows to run out of feed and then allowing them to eat large quantities of highly soluble carbohydrates, which can again disrupt the microbial population in the rumen. In times of drought, cows on pasture may not receive adequate quantities or quality of forage but may receive over 25 pounds of concentrate mix per day. In such situations, cows may be acidotic and show milk fat depression. If the cows have acidosis for an extended time, they may have damage to the rumen, which limits their ability to produce milk for several months.

In the summer months, cows will decrease total consumption of feed as environmental temperatures rise. As a result of eating less feed, cows will often have less intake of forages because forages have a higher heat increment than grains and this, too, may contribute to a low fat percentage in the milk. In the spring and fall of the year, cows that are grazing on wheat, ryegrass or other fast growing grasses also may have a low fat percent because of high levels of soluble carbohydrates in the diet. Usually, these diets can be adequate, if not superior to most, for the cow if buffers are added to the concentrate mix, and long-stemmed hay is available for the cows to eat.

Other causes of a low fat test in the milk are less common but include inadequate levels of rumen ammonia, so that microorganisms may not be properly functioning, and poor mixing of a total mixed ration (TMR) such that some cows receive fine particles and other cows receive the more coarse particles of feed. Having dead spots in your mixer wagon can result in undermixing portions of the ration and overmixing other portions, which again results in sorting of the ration with finer particles being distributed to some cows and coarser particles to other cows. Cows receiving the finer particles are more apt to have a lower fat percentage in milk because the feed is more quickly attacked by the microorganisms and these cows are more apt to have lower rumen pH shortly after feeding as the microbes produce acid (volatile fatty acids) when they metabolize the feed.

Methods to Maintain Fat in Milk

For most dairy producers, the most effective method of maintaining an adequate milk fat percentage in the milk is:

     • ensure that adequate fiber is in the ration, and
     • include buffers in the concentrate mix or total mixed ration of the cow (Table 1).

Table 2 shows generally recommended levels of sodium bicarbonate and magnesium oxide that are included in the ration. If a herd has a deficiency of forage or fiber in the ration, the deficiency needs to be corrected as soon as possible to avoid damage to the anatomy of the cow. However, it is also important to recognize that adding buffers to a ration generally costs three to five cents per cow per day and may not be necessary if the ration is properly balanced and fed.

Conditions for best results with buffers include feeding limited quantities of long-stemmed hay to the cows, usually at least 5 pounds per cow per day, and maintaining a stable diet to the cow with no abrupt changes in the feed. Other economical methods of feeding to prevent low fat milk in herds include adding 2 to 3 pounds of cottonseed hulls or 4 to 6 pounds of soybean hulls per cow per day to stabilize the rumen and to ensure that there is enough effective fiber in the diet.

When the fat in milk is above 3.2 percent, changes in the diet of the cow to increase fat percentage in the milk may or may not be economical. Some helpful hints in terms of increasing fat percentages if the fat percentage is above 3.2 percent are:

     • Feed hay before grain or a concentrate mix when feeding the two ingredients separately.
     • Feed four to six times per day to ensure that the microorganisms in the rumen have a steady diet of
       feed available to them.
     • Always have long-stemmed hay available to the cows.

Additional methods to slightly increase fat percent in milk when the fat is above 3.2 percent is to feed whey or B vitamins in adequate quantities; feed bentonite, sodium bicarbonate or sodium sesquicarbonate free choice; and provide adequate water to the cow to ensure that she has stable metabolism within the rumen. Feeding such ingredients as whole cottonseed, cotton hulls, soybean hulls, methionine hydroxyanalog (MHA) or whey will often also raise fat percentage, especially if the fat is slightly depressed and the depression is related to feeding of the dairy cows.

For most dairy producers, the addition of buffers to the concentrate mix or total mix ration is an economical method to maintain fat percentage when the ration of the cow is changing often. For example, if a cow is on pasture with lush grass and the grass is maturing or decreasing in availability or if there is any irregularity in the feeding, usually buffers in the ration will result in an increase in fat percentage within 7 to 14 days if the milk fat is less than 3. Moreover, the addition of buffers to the ration may increase milk production slightly from 4 to 10 percent as it increases milk fat percentage by 2 tenths to 4 tenths of a percent if the milk fat percentage is at or below 2.8 percent fat. In cases of proper feeding with a stable rumen environment, the addition of buffers will probably not increase fat percentage but may allow the dairy producer to have some insurance against fluctuation in the diet contributing to an unstable rumen environment.

Summary

The best method to maintain an adequate fat percentage in milk is to feed a balanced ration with adequate forage. Cows should have access to long-stemmed hay at all times and buffers should be included in the ration if cows are changing feeds as pastures change or if cows are getting greater than 12 pounds of concentrate as they go through the milking parlor. Regular feeding of a clean, dry forage and grain mix is critical to maintaining cows on feed and a stable rumen environment. Other feed additives such as whole cottonseed, cottonseed hulls, soybean hulls or small amounts of feed such as methionine hydroxyanalog (MHA) also may benefit the milk fat percentage in some dairy herds. It is also possible to select cows for higher fat percentage in the milk or to breed cows to a sire with a high-predicted transmitting ability for fat, so that his daughters will have a higher fat percentage in the milk than their mothers.