A closer look at monensin inclusion levels in feedlot diets

The use of monensin is a commonly used in feedlot diets in Canada to stabilize dry matter intake and improve feed conversion. In the past, label claims included inclusion rates up to 33 ppm; however, more recently the Canadian Food Inspection Agency increased the maximum inclusion level for feedlot cattle to 48 ppm. The ionophore monensin works to alter the rumen microbial population changing the predominant short-chain fatty acids (SCFA) that are produced by the rumen microbial populations. The resulting shift in SCFA profile increases propionate at the expense of acetate, resulting in greater nutrient capture from the feedstuff. While numerous studies have established the impacts of monensin at traditional inclusion rates of 22 or 33 ppm, very few studies have investigated beef cattle responses at higher monensin doses.

Recent work from the University of Saskatchewan, published in the Journal of Animal Science, took a closer look at the impact of monensin inclusion levels on total tract digestibility, rumen SCFA concentration, and SCFA absorption from the rumen. In a Latin Square experiment, four Hereford crossbred ruminally cannulated heifers were fed a high grain finishing diet (76% barley grain, 12% barley silage) and offered either 0, 22, 33, or 48 ppm of monensin.

Feed intake, performance and digestibility

Throughout the experiment, heifer bodyweights remained similar between the different monensin inclusion levels. Individual feed intakes measured at the end of each feeding period revealed that intakes decreased with increasing monensin concentration (Table 1) including decreased dry matter intake at the 48 ppm inclusion level. There were also numerical decreases in the day-to-day variation in feed intake with those fed 48 ppm monensin having the least variation in intake during the measurement period. This suggests that perhaps using monensin at the higher dose rate may be useful to producers at times when intakes may be highly variable, as variable feed intake can increase risk for ruminal acidosis and poor feed efficiency.

Nutrient digestibility is an important consideration in maximizing efficient feedlot production and reducing nutrient loss. In this study, overall total tract digestibility was not impacted by monensin concentration. However, ethanol soluble carbohydrate (simple sugars) digestibility, increased linearly with increasing monensin inclusion level. Although the reason why minor changes to ethanol soluble carbohydrate digestibility occur is not clear, overall increased monensin inclusion did not negatively impact total tract digestibility.

Table 1: Body weight, feed intake, rumen pH and short-chain fatty acid (SCFA) concentration in heifers fed a high grain diet differing in monensin concentration

 
Dietary Treatments
P-value
0 ppm
22 ppm
33 ppm
48 ppm
SEM
Linear
Quadratic
Body weight, kg
358
347
347
357
10.2
0.82
0.25
Dry matter intake, kg/d
10
10
9.3
9.1
0.35
0.012
0.37
Standard deviation in dry matter intake, kg/d
0.93
0.62
0.48
0.47
0.213
0.13
0.6
Total tract digestibility, %
79.4
77.3
76.3
78.8
1.62
0.60
0.17
Mean rumen pH
5.83
6.03
5.75
5.88
0.125
0.97
0.66
Total rumen SCFA concentration, mM
132.7
128.4
133
132.2
3.88
0.93
0.51
Acetate, %
49.9
49.4
49.2
48.5
1.79
0.59
0.91
Propionate, %
31.2
31.9
33.5
35.9
3.41
0.35
0.73
Butyrate, %
14.7
13.3
13.7
12.7
1.75
0.54
0.95

Rumen pH, Short Chain Fatty Acid (SCFA) concentrations and absorption

Low rumen pH can be an indication of ruminal acidosis and result in poor feed conversion and poor performance over the finishing period. Mean rumen pH was similar amongst all monensin inclusion levels, indicating that monensin did impact rumen pH. Rumen SCFA concentrations (acetate, butyrate, and propionate) also did not differ with monensin inclusion levels. However, ratios of acetate to propionate did improve linearly with increasing monensin inclusion levels (Figure 1). This indicates that the new higher monensin inclusion levels are consistent with known impacts on rumen fermentation and may offer further improved feed utilization, however this needs to be verified in additional experimentation.

Rumen acetate to propionate ratio from heifers fed differing concentrations of monensin

Figure 1: Rumen acetate to propionate ratio from heifers fed differing concentrations of monensin

Text equivalent to Figure 1

One aspect that was not known is how monensin impacts the absorption of SCFAs in the rumen. This is important as SCFA are the primary soure of energy for beef cattle. Short-chain fatty acid absorption can be measured in ruminally cannulated cattle using the washed-rumen technique. For this technique, the rumen is emptied and washed free of digesta and the esophageal and omasum openings are temporarily plugged. Then a warm solution with a known SCFA concentrations is placed into the rumen, continually mixed, and the solution is sampled over time to determine disappearance of SCFAs from the solution, which equates to SCFA absorption across the rumen (Figure 2). The absorption rate of SCFAs (acetate, butyrate, propionate) was not different among all monensin inclusion levels (Figure 3), indicating that absorption of SCFAs was not significantly impacted by monensin concentration.

The temporary isolated washed rumen technique for measuring absorption of short-chain fatty acids (SCFAs) from the rumen

Figure 2: The temporary isolated washed rumen technique for measuring absorption of short-chain fatty acids (SCFAs) from the rumen

Text equivalent to Figure 2

Rumen absorbance as determined using the washed rumen technique from heifers fed differing concentrations of monensin

Figure 3: Rumen absorbance as determined using the washed rumen technique from heifers fed differing concentrations of monensin

Text equivalent to Figure 3

Should you consider using increasing to 48 ppm monensin?

As there were limited negative impacts on the animal in increasing to the new 48 ppm inclusion, the decision to increase to 48 ppm monensin in the ration may come down to an economiclly based one and will these new inclusion levels have a positive return of investment in commercial feedlots. As this study was a small number of animals, it was not possible to evaluate overal growth performance and feed efficiency measurements at the higher inclusion rate. However, If monensin costs are aproximatly 6 cents per day at 33ppm, an increase to 48 ppm is roughly 8.75 cents per day. If feed cost of gain is assumed at $1.6 kg gain/d, an approximate 2% improvement in feed conversion is needed to break even. In addition, with observed decreases in feed intake and a potential for reduced variation in feed intake, including monensin at 48 ppm may help to normalize feed intake, which may positively impact rumen health.


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