Maintaining desirable milk fat (MF) levels on a consistent basis is one of the most frustrating challenges facing dairy nutritionists and consultants today.
Historically, milk fat has been used as a measure of rumen health and a low milk fat test was often thought to be associated with rumen acidosis. This perceived relationship has often placed the blame for a low milk fat test (less than 3.5 percent) on diets with excessive amounts of highly fermentable carbohydrates. The basis for this idea was two-fold: acetate being an important carbon source for milk fat synthesis and existing data showing a decrease in acetate-to-propionate ratios when cows were fed a diet high in concentrate and low in roughage. Subsequent research, however, indicated that the reduction in the acetate-to-propionate ratio was due to an increase in propionate rather than a decrease in acetate, thus acetate was not limited.
More recent research has led to a much more unifying theory of milk fat depression. It suggests that milk fat synthesis is inhibited directly at the mammary gland level. This is called the biohydrogenation (BH) theory, which was proposed by Bauman and Griinari in 2001. This theory states that “under certain conditions, rumen biohydrogenation results in unique fatty acids that are potent inhibitors of milk fat synthesis (e.g., trans-10, cis-12 conjugated linoleic acid [CLA] and possible related intermediates from linoleic acid and other polyunsaturated fatty acids [PUFA]).”
The BH theory has helped to explain many situations in which rumen health appears to be excellent and fermentable carbohydrate levels in the diet are well within acceptable ranges, but milk fat is still an issue. This theory further reveals that milk fat is a multifactorial issue that is dependent on management and nutritional decisions that can affect rumen biohydrogenation.
Nutrition and management decisions can contribute to an increased risk for milk-fat-depressing biohydrogenation intermediates passing out of the rumen (see Figure 1*):
1. By increasing the supply of substrate (PUFA) for the formation of CLAs, noted to cause milk fat depression
2. By altering the rumen in a variety of ways, impacting biohydrogenation
3. By affecting the rate of biohydrogenation with certain dietary components that increase the passage of biohydrogenation intermediates out of the rumen, some of which contribute to milk fat depression Rate of movement of feedstuffs through the rumen can also increase the likelihood of biohydrogenation intermediates passing through the rumen.
Troubleshooting milk fat issues remains one of the most persistent and challenging tasks in managing the nutrition of a dairy herd.
1. The first step is to identify potential risk factors for milk fat depression based on when the milk fat problems started. Evaluate herd history, such as production and component trends during a typical year. Look at changes in dry matter intake, as well as recent changes in diet or management.
2. The next step is to pinpoint what risk factors for milk fat depression are present on the farm. Consider how each may contribute to the amount of polyunsaturated fats (PUFA) in the diet, how they may alter the rumen environment and finally how those factors may change the rate of biohydrogenation.
When determining factors that may contribute to the amount of PUFAs that the rumen is being exposed to at any one time, both the amount and availability of the PUFAs in these feed sources should be taken into account. In assessing the availability of PUFA, include processing of the feed ingredient (i.e., ground versus whole seed), moisture content of the feed ingredient (wet distillers versus dry distillers grains) and the potential variability of the fat content in any particular feedstuff. Feedstuffs with the highest content of PUFAs at the highest degree of availability put the herds at greatest risk for milk fat issues.
As noted in the introduction, rumen environment is often the initial consideration when troubleshooting milk fat and, more specifically, the fermentable carbohydrates in the diet and their potential effect on the environment. Low rumen pH can indeed impact the rumen environment and also increase the risk for milk fat depression, but this is just the tip of the iceberg. Management and nutritional decisions that can affect the rumen environment encompass a variety of sources, ranging from the total ration to individual ingredients to facilities to cow factors.
When examining diet and individual ingredients, consider everything from diet formulation and mixing uniformity to quality of fermented feeds, carbohydrate sources, physically effective fiber and, again, unsaturated fatty acid sources, which will also lead to a change in the rumen environment.
Facilities and cow factors that should be reviewed include anything that may impact sorting, feed intake patterns, cow comfort, cooling, stocking density and water intake.
3. The final risk area to examine is whatever might influence rates of biohydrogenation or the rate of passage. One primary consideration when investigating risks of slowing the rate of biohydrogenation is specific unsaturated fatty acids, such as fish oil. Herds with higher DMI are at a greater risk of increased rate of passage, as are those with rations containing less effective fiber or herds more prone to slug feeding and sorting.
When evaluating the dietary portion of these risk factors, it is important to evaluate both the expected diet and, probably more so, the actual diet being consumed by the cow. Focus more attention on things such as mixing, sorting and other factors that could result in the difference between the formulated diet and the consumed diet.
When battling milk fat challenges on the farm, always remember that milk fat depression is a multifactorial issue. Therefore, it is unlikely that the same risk factor is responsible for depressing milk fat in multiple situations, and it is likely that there is more than one factor contributing to the milk fat issues.
A recent epidemiology study completed at Cornell University evaluated the effect of dietary and management practices on bulk tank milk fat level. The study showed that no single TMR characteristic, management practice or ration component accounted for more than 10 percent of the variation in herd level milk fat, helping solidify the idea of milk fat depression as a multifactorial issue.
Finally, keep in mind that the rumen is a dynamic place and changes in management or nutrition decisions may take a minimum of two to three weeks before bulk tank milk fat content recovers. PD
*References and figures omitted but are available upon request at editor@progressivedairy.com
Chel Moore
Tech. Consultant
Elanco Animal Health
moorece@lilly.com
