September 5, 2017 at 3:32 p.m.

Research answered dairy questions

By Jim Linn- | Comments: 0 | Leave a comment

One exciting and rewarding aspect of working with dairy farmers is they always ask questions. Dairy farmers have creative minds and look for best management practices to implement even when what they are doing is working well. Over the years, there were many times I did not know of a research study to provide a direct answer to the question asked, but through dialog and recalling some related research a plausible answer often was arrived at. So, when research studies appear later that directly pertain to a question asked, it is always exciting to see if the research finding supports the answer arrived at. Below are a few questions that have been recently asked with research reported in the December (2012) and January (2013) issues of the Journal of Dairy Science (JDS) addressing the question.

If acetic acid is a precursor for milk fat synthesis, could a person feed acetic acid (vinegar) to cows to boost milk fat, and how much would you have to feed?
Reported in the January issue of JDS is a research study from the University of São Paulo in Brazil on feeding a diet containing 5 percent acetic acid to mid-lactation cows. The basal diet contained 33 percent forage (chopped Bermuda hay) and 67 percent concentrate and was fed as a TMR. One complication in conducting this type of research is almost all moisture testing procedures involve heat which removes volatile fatty acids along with the water. Acetic acid is an organic compound and easily vaporized during the drying of feeds. This results in the moisture content of the feed being over estimated or underestimation of the true dry matter of the feed. Using all dry, unfermented feeds in this study helped avoid the problem and the researchers added the 5 percent acetic acid to the oven DM of the TMR to determine the actual daily DM intake of cows.
Adding 5 percent acetic acid into the TMR at feeding depressed DM intake the first two weeks of feeding, but by four weeks into the study, the deleterious effects on DM intake disappeared. Milk production was unaffected by feeding acetic acid. Even during the first two weeks cow adapted to the acetic acid diet, both control and acetic acid fed cows averaged 78 lb/day. Cows fed the acetic acid diet averaged 0.19 percent higher fat test than control cows although the difference was not statistically significant. Feed efficiency was slightly improved in acetic acid fed cows (1.66) compared to control cows (1.61), but digestibility of the diet was not affected.
To duplicate this research, you would have to feed 50 pounds of vinegar (5 percent acetic acid) or about eight gallons/cow/day at 50 pounds of DM intake.

Will feeding polyunsaturated fatty acids help improve my reproductive performance?
Two recent studies with feeding polyunsaturated fatty acids (PUFA) show they have to be rumen inert to avoid milk fat depression and some measures of reproductive performance improved, but not all.
In the first study from Cornell University in the December 2012 issue of JDS, researchers fed a diet containing 2.5 percent soybean oil (DM basis) in an attempt to increase the conjugated linoleic acid (CLA) content of milk. As you may recall from previous columns, CLA is a bioactive fatty acid that is anticarginogenic and may be beneficial against chronic diseases. While soybean oil is a good source of PUFA, and can potentially increase the healthy CLA content of milk, if not protected from biohydrogenation in the rumen the bacteria will transform it into other forms of CLA fatty acids leading to milk fat depression. Cows fed the 2.5 percent soy oil diets produced milk containing 2.94 percent fat compared to 3.53 percent fat in the milk of control cows. Milk production was not different between control fed cows (85 lb/day) and those fed soy oil (81 lb/day). In an attempt to alleviate the milk fat depression caused by PUFA, 10,000 IU/day of vitamin E was added to the soy oil diet. Cows fed the soy oil plus vitamin E responded in milk (92 lb/day), but milk fat remained depressed ( 2.92 percent).
The effects of feeding a rumen protect source of PUFA on reproduction was reported in the December issue of JDS by University of São Paulo in Brazil dairy scientists. They conducted two experiments with over 1,100 cows in experiment one and 1,500 cows in experiment two. The PUFA diet contained a calcium salt of a PUFA product at 1.1 percent of the DM. The fatty acid content (reported as percent of total fatty acids) of the control and PUFA diets were respectively; linolenic acid (18:3) - 4.2 vs. 5.1 percent, oleic acid (18:1) - 17.8 vs. 21.1 percent and linoleic acid (18:2) - 52.8 vs. 49.2 percent.
Cows fed the diet containing PUFA produced more milk (83 lb/day) with a lower fat test (3.41 percent) than cows fed the control diet (78 lb/day and 3.55 percent fat). The milk yield response came through a greater persistency over a 43 week lactation and not as peak milk yield. Dry matter intake was unaffected by feeding PUFA and averaged 52 lb/day for both groups.
Overall reproductive performance was not statistically improved as measured by pregnancy rates (45 vs. 48 percent for control and PUFA treatments, respectively) and services per conception (2.22 for control vs 2.05 for PUFA, respectively). However, as part of experiment two some cows were not fed the PUFA supplement until 30 days in milk. Cows receiving the PUFA supplement beginning at 30 days in milk had fewer pregnancy losses to first service (15 percent) compared to cows receiving the supplement immediately after calving (32 percent), with less total pregnancy losses per service favoring the PUFA supplemented cows. Also interesting was there was more benefit to feeding the total amount of PUFA supplement once per day in the A.M. TMR mix rather than equally dispersed in six feedings of TMR per day. Speculation is feeding once per day resulted in less dissociation and less biohydrogenation of the PUFA supplement resulting in more PUFA available post-ruminally then when fed six times per day.

What is a good value for milk urea nitrogen (MUN), and why have my MUNs increased 1 to 2 mg/dl in the last month when no feeding changes occurred?
Establishing a MUN baseline for individual herds and evaluating changes in MUN from the individual herd data is the best way to assess changes in MUN. Research by Virginia Tech dairy scientists in the December issue of JDS re-established that protein feeding, neutral detergent fiber or concentrate levels in the diet along with stage of lactation, milk production and milk composition all have a significant effect on MUN. However, they also found individual cow variation within herds to be a significant factor in herd MUN level. Phenotypic difference in MUN among cows was quite large and could not be explained by diet, milk production or other management factors. Some cows are genetically programmed to secrete more nitrogen into milk than urine which supports why establishing a MUN baseline for an individual herd is important. The MUN concentration in milk, on which this research was based ran slightly higher (12 - 21 mg/dl) than what we would consider normal for cows or herds. Typically, we want to see MUNs between 10 and 14 mg/dl with less than a 1 mg/dl variation between bulk tank pickups or DHIA tests.
In the same December issue of JDS was another article on sodium intake and MUN. Researchers from the Netherlands fed 0.04, 0.20, 0.36 and 0.51 percent salt in lactating cow diets to achieve intakes of 69, 198, 292 and 417 g/day of sodium. What they found was urine volume increased and MUN concentration in milk decreased with increasing amounts of sodium consumed per day. Cows fed the lowest sodium amount had a 13.3 mg/dl MUN concentration in milk compared to 10.8 mg/dl for cows fed the highest amount of sodium per day. Urine volume increased with increasing sodium intake as cows excreted the extra sodium to try and balance their electrolyte load. Urea being water soluble and having osmotic properties increased in excretion as urine volume increased. Urine volume was found to be highly correlated with sodium intake per day: Urine (kg/day) = 7.5 +.136 x sodium intake (grams/day).
While these two studies do not directly answer why MUN increased in several herds the past few weeks, they do provide more insight into the variability of MUN and why cows and herds do not all have the same urea concentration in milk. We do know MUN is related to the amount and form of protein in the diet and with colder weather cows consume more feed. The increase in feed/protein consumption will increase blood urea concentrations and if slightly less water is consumed in cold weather, urine volume will decrease leaving more urea circulating in the blood to be transferred into the mammary gland and excreted in milk.
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