There has been substantial focus, both in the scientific literature and from anecdotal reports mostly from Canada, about the perceived benefits from feeding ionophores to cows during the transition period, a practice that has not been approved by the FDA in the United States. The two major ionophores approved for use in diets fed to growing heifers are monensin and lasalocid. The majority of research focusing on the transition cow has been conducted using monensin. A series of papers based upon a commercial experiment and a university herd experiment by workers at the University of Guelph examined whether monensin supplementation would decrease incidence of ketosis (presumably through increasing propionate supply to the liver for use in glucose synthesis) and enhance milk yield during early lactation.

The commercial experiment involved a total of 1010 Holstein cows and heifers from 25 cooperating farms in Ontario. Animals were administered an intraruminal controlled release capsule containing either a placebo or monensin designed to release approximately 335 mg per day for approximately 95 days. Concentrations of b -hydroxybutyrate in serum were significantly reduced during weeks 1, 2, and 3 postpartum, and serum glucose concentrations were significantly increased during weeks 1 and 2 postpartum (Duffield et al., 1998. J. Dairy Sci. 81:2354-2361). These workers also reported that cows administered monensin lost less body condition during early lactation, although these differences averaged less than .05 of a condition score. Reanalysis of the same dataset using a threshold for subclinical ketosis of 1200 m mol/liter of b -hydroxybutyrate in serum indicated that subclinical ketosis was reduced from approximately 30% of cows to 15% of cows at two weeks postcalving (Duffield et al., 1998. J. Dairy Sci. 81:2866-2873). Results of serum ketones and milk ketones corresponded well with one another, but (not surprisingly) the milk ketone tests were not as sensitive as the serum analyses. These data supported previous data reported in abstract form (Thomas et al., 1993. J. Dairy Sci. 76(Suppl. 1):280.(Abstr.)), which demonstrated decreased concentrations of NEFA and b -hydroxybutryate during the first 28 d postpartum in cows fed monensin.

The data for milk yield during the first three test days of the cows in this experiment were presented in a third paper (Duffield et al., 1999. J. Dairy Sci. 82:272-279) and are shown below:

Unlike previous reports (Sauer et al. 1989. J. Dairy Sci. 72:436-442) that milk fat and milk protein percentage are decreased when monensin is fed to cows during early lactation, milk fat and milk protein percentages were not affected by treatment in this experiment.

These data make sense biologically. First, thin cows that are less susceptible to metabolic problems after calving did not respond to monensin in terms of milk yield. Cows in moderate condition responded only at the second test day, and this effect did not carry over to the third test day. Fat cows that are more susceptible to postpartum metabolic problems responded well at all three test days. Although the authors did not discuss the body condition score effects independently, it appears from the table above that cows that were categorized as thin (76.0 lb/day) and fair (75.6 lb/day) averaged more milk during the first three test days than those that were fat (74.1 lb/day) at three weeks prepartum. These data support our current recommendation to dry cows off at a body condition score of 3.25 to 3.50 and neither gain nor lose body condition during the dry period.

The researchers did not make the following measurements, but we would think that the thinner and moderately conditioned cows ate more postcalving, mobilized less body condition, and thus had decreased concentrations of nonesterified fatty acids (NEFA) in blood. Because the liver would have taken up fewer NEFA, the processes of oxidation and lipoprotein export would not have been overloaded and fewer triglycerides would have accumulated in liver (Figure 1). Because dry matter intake (and glucogenic precursor supply) would have been greater, fewer ketone bodies would have been produced because NEFA would have been more completely oxidized (Figure 2).

In a subsequent experiment conducted at the University of Guelph, Green et al. (1999. J. Dairy Sci. 82:333-342) administered the same intraruminal controlled release capsule containing either a placebo or monensin at three weeks prepartum to 11 heifers and 41 cows. Body condition scores averaged 3.65 and 3.60 for heifers and cows, respectively, during the last two weeks prepartum. Diets were fed for ad libitum intake during the last two weeks prepartum and the first two weeks postpartum, then restricted to 90% of ad libitum intake from weeks 3 through 5 postpartum before returning to ad libitum intake during week 6 postpartum. Dry matter intake and blood parameters during the two weeks prepartum were minimally affected by monensin. Dry matter intake during the 6-week postpartum period was not affected by monensin treatment, but monensin decreased concentrations of b -hydroxybutyrate in serum and increased concentrations of glucose in serum during the 6-week postpartum period. Heifers and cows responded similarly to monensin treatment during the 6-week postpartum period.

The lack of a response to monensin during the two weeks precalving makes sense. The cows and heifers were consuming approximately 17 and 25 lb per day of dry matter during the last two weeks prepartum. Given that the demands for glucose and reliance on body fat stores for energy are much less before calving than after calving, one would not expect to see a response to monensin before calving unless there were major problems with intake or energy imbalances in the closeup ration. The only rationale for including monensin in rations for closeup cows is to allow time (~ 3 weeks) for the rumen to adapt to monensin.

This experiment has some design considerations that must be kept in mind when evaluating these data. First, dry matter intake was restricted during a large portion of the postpartum period and averaged 24.5 lb per day for heifers and 32 lb per day for cows. Cows in well-managed herds will average 45 lb or more of DMI per day and heifers will average 35 lb or more of DMI per day during the first 6 weeks after calving. These researchers demonstrated that supplementation with monensin improves metabolic health when dry matter intake is restricted to levels substantially below that of well-managed commercial herds. The true question is whether monensin improves metabolic health when fed to well-managed cows, and treatments to determine the answer to this question were not included in this experiment.

• There is little metabolic data to support including ionophores in diets for closeup dry cows. The only rationale for including them in diets fed to closeup cows is to allow time for the rumen environment to adapt before lactation.
• Cows that are excessively conditioned (> 3.75) during the middle of the dry period, and already at much greater risk for metabolic problems, are likely to respond to ionophore supplementation during early lactation.
• Cows that are moderately conditioned or thin probably will not respond to ionophore supplementation, although experimental data that are more typical of well-managed herds are not available.
• In summary, after they are approved for inclusion in rations for lactating cows, ionophores will be beneficial when targeted for use in herds with problems with overconditioned dry cows, but responses in well-managed herds where body condition is moderate and cows are not at great risk for metabolic disorders probably will be minimal.

T. R. Overton

Mark your calendars for October 19, 20, 21, 1999. These are the dates for the 1999 CNC to be held at the Marriott Thruway Hotel in Rochester. Dan Brown and the program committee is finalizing speakers and topics. Breakfast meetings will be sponsored by Zinpro and Alltech. Registration materials will be mailed out in August. Please contact Dee Brothers (dab2@cornell.edu) or Larry Chase (lec7@cornell.edu) if you have any questions. Our FAX number is: 607-255-1335. See you in Rochester!

The Dairy Professional program will offer both Basic and Advanced Dairy Nutrition courses for feed industry professionals in 1999. Both courses were filled in 1998 and we were pleased with the positive feedback from participants. Dates and locations for 1999 are: