By Dr. Dan Schauff, Ph.D., P.A.S.
Despite advances in nutritional and feeding knowledge over the years, milk fat depression can still be a problem on dairy farms. Traditionally, when milk fat declines in a herd, we start with verifying that the “paper” ration equals what the cows consume. Then we make sure the ration is supplying adequate forage and effective fiber while avoiding too much rapidly fermented corn or starch. However, in some cases, inadequate fiber or too much starch is not the main cause of milk fat depression. Most nutritionists and dairy producers are careful to formulate and feed rations that optimize rumen and cow health, but why do we still struggle with milk fat depression in some situations?
Origins of Milk Fat:
Normally, about half of milk fatty acids are produced in the mammary gland from acetate and butyrate generated from rumen fermentation (Palmquist and Jenkins, 1980). Acetate is produced mainly from forage/fiber fermentation in the rumen, hence the importance of fiber digestion on supplying acetate for milk fat synthesis. About 40 to 50% of milk fat comes from dietary sources of fat and less than 10% of the cow’s mobilized fat stores (Palmquist and Jenkins, 1980). However, early lactation cows in negative energy can have a higher proportion of milk fat coming from their fat stores. It is generally believed that milk fat depression occurs when mammary milk fatty acid production (de novo synthesis) is inhibited or reduced.
The Cause of Milk Fat Depression:
The depression of milk fat synthesis in the mammary gland starts in the rumen. Most feed sources of fat for dairy cows in both forages and concentrate ingredients contain unsaturated fat and fatty acids. Because unsaturated fatty acids contain one or more double bonds along their carbon chain, they are liquid at room temperature (70° F). These liquid unsaturated fatty acids are “active” and can be toxic to some rumen bacteria. The most abundant dietary source of unsaturated fatty acid is linoleic acid (C18:2) from corn, soy, and other oil seeds. To detoxify linoleic acid and other unsaturated fatty acids they undergo biohydrogenation by rumen bacteria to mainly stearic acid (C18:0) which is a saturated fatty acid that is inactive and inert in the rumen. When the rumen microbial population is normal and stable, most of the linoleic acid is converted to stearic acid (C18:0) which results in normal milk fat (See Figure 1). However, when the normal rumen microflora is altered or disrupted, an alternative biohydrogenation pathway can occur (See Figure 1) resulting in fatty acid intermediates being formed in the rumen such as trans-10, cis 12 conjugated linoleic acids (CLA) (Figure 1). The ruminal production of as little as 1 to 2 grams of the aforementioned CLA isomer can directly inhibit milk fat production in the mammary gland (Lock, 2009). Consequently, the direct cause for milk fat depression is straightforward, however, the indirect cause(s) of a disrupted/altered rumen environment is more complicated.
Causes for an Altered Rumen Environment:
There are numerous nutritional/management related factors that can create this altered rumen environment that leads to an increase in ruminal production of CLA isomers that decrease milk fat synthesis. Ration related factors include feeding too much corn or starch, too little “effective” fiber (aforementioned), too much rumen active fatty acids (RAFA) like linoleic acid (C18:2) and the presence of molds/mycotoxins and wild yeast in feeds. The impact of excessive starch and low fiber in rations on altering the rumen environment is well known and will not be discussed further. We also know from field experience that herds tend to struggle with lower milk fat when feeds are higher in mycotoxins and “wild” yeast.
In the last 10 to 15 years we have gained a better understanding of the role of dietary linoleic acid (C18:2) on rumen fermentation and milk fat depression. Rations containing over 400 grams/head/day of linoleic and other rumen active fatty acids (RAFA) can result in milk fat depression (See Table 1). This may be due to a direct effect on the rumen microbes or an overload of the normal biohydrogenation pathway (See Figure 1) resulting in more CLA isomers being produced. Rations that contain too much oil and RAFA from corn distillers, roasted beans and/or whole cottonseed may reduce milk fat (See Ration 2; Table 1). However, even rations that appear to contain conservative amounts of oilseeds, and starch and adequate “effective” fiber can be at risk for lower milk fat as well(Ration 3; Table 1). For example, a ration that contains a higher amount of corn silage where oil/RAFA levels in the corn silage and corn grain is above average can also exceed 400 grams of RAFA and thereby result in a decrease in milk fat. (See Ration 3; Table 1). Therefore, monitoring and controlling RAFA levels in rations is important to minimize the risk of milk fat depression.
Feeding Rumensin® can be a tool to improve feed efficiency with minimal effects on milk fat concentration. However, feeding Rumensin® along with too much RAFA can decrease milk fat even more than feeding too much oil or Rumensin® by themselves (Alzahal et al., 2008).
There are numerous feeding management and cow environmental factors that can alter the rumen environment which include: feeding accuracy/consistency, feed sorting, feeding frequency, feed bunk space, stocking density, heat stress, and cow comfort. In a field study conducted in New York State and Vermont from 44 commercial dairy farms, it was observed that herds with higher milk fat tended to feed more frequently, have greater bunk space, and lower stocking density (Woolpert et al., 2016). Ration nutrient concentrations were not greatly different between the higher milk fat herds versus the lower milk fat herds (Woolpert et al., 2016).
To minimize the risk for milk fat depression, consider the following management/nutritional guidelines.
1) Provide heat stress abatement in the summer such as shade, fans and sprinklers in the holding area and feed bunk line.
2) Optimize free stall space, bedding management, and neck rail placement to encourage stall use and cow comfort.
3) Avoid free stall barn overcrowding (if possible, avoid much more than 105-110% stocking density).
4) Supply plenty of bunk space (24-30 inches/cow).
5) Feeding frequency:
a. Free stall barns (TMR Feeding) – Consider mixing and feeding twice daily to minimize the effects of cows sorting and “slugging” concentrate. Push-up often during the first 2 hours after each feeding (once every 30 minutes).
b. Tie Stall Barns (Component Fed) – Consider feeding grain 3 to 4 times/day. Offer hay or balage an hour or two before each grain feeding.
6) Conduct regular TMR mixing audits
a. Make sure scales are accurate when the mixer is full and empty.
b. Replace mixer knives as often as needed to ensure adequate and consistent processing of long particle forages.
c. Evaluate the sequence of adding ingredients and mixing time to ensure a uniformly mixed TMR from the beginning to the end
of the mix.
d. Consider adding water if the TMR moisture is below 45% (48-50% moisture is ideal) to minimize feed sorting.
e. Take TMR samples often (monthly) to confirm that the ration “on paper” is close to the ration delivered.
f. Evaluate TMR particle size using the Penn State particle size separator to ensure the ration is supplying enough “effective fiber”
but not too much “long fiber” that can be sorted.
7) Avoid feeding moldy/spoiled/heating feeds that may contain high levels of yeast and mycotoxins.
1) Starch – Maintain starch levels between 22 – 28%. If the starch source is from a well processed and fermented corn silage,
snaplage and HM corn, ration starch concentration may need to be kept in the lower 20’s %. If the starch source is from poorly
processed and less well-fermented corn silage and grains, higher starch levels can be tolerated even exceeding 28% in some cases.
2) NDF/NDF from forage – To maintain a good rumen “mat” for optimal digestion and cud-chewing maintain 28 – 32% NDF and
20 – 26% NDF from forage.
3) Rumen active fatty acids (RAFA) – Have your nutritionist monitor RAFA levels in rations to avoid more than 400 grams.
4) Mineral/electrolyte balance – Make sure ration minerals are balanced properly to meet the cow’s requirements and provide
the buffering capacity to the ration especially during the summer. AK
*References available upon request.