Supplementing Monensin and Rumen Undegradable Protein to Late-Gestation Heifers

Authors: Dempster M. Christenson, Research Technician; Rick N. Funston, Professor, WCREC, North Platte.

Summary with Implications

This study evaluates the effects of supplementing late-gestation heifers with rumen-undegradable protein and monensin on weight gain, feed efficiency, and reproductive performance, and subsequent impact on progeny. We hypothesized that supplementation would enhance growth and feed efficiency, ultimately improving cow and calf performance. Previously synchronized artificially inseminated heifers were fed ad libitum hay for 89 d prior to calving. Feed was top dressed with chelated mineral with or without monensin, dried distillers’ grains, or dried distillers’ grains and monensin. Supplementation with rumen undegradable protein increased growth rate and decreased calving difficulty with no further effects on cow or calf performance.

Introduction

Winter management of pregnant spring-calving heifers is critical to support their continued growth and development. During late gestation, heifers are still maturing while also supporting a growing fetus that demands nutrients and can limit rumen capacity. Compounding this challenge, their diet often consists of low-quality roughage sources during the winter months. Thus, as heifers approach their first calving , they must balance the physiological demands of parturition, lactation, and continued development.

A protein source, like dried distillers’ grains, high in rumen undegradable protein (RUP) is primarily absorbed by the small intestine, providing essential nutrients directly to the animal with minimal degradation by ruminal bacteria. Monensin is an ionophore fed to cattle to prevent coccidiosis but also shifts ruminal fermentation patterns impacting feed efficiency, rumen feed passage rate, and protein utilization. Both nutrient restriction and supplementation of gestating heifers affects future growth, reproductive efficiency, and offspring growth. The objective of this study is to evaluate the effects of supplementing late-gestation heifers with rumen undegradable protein and monensin on weight gain, feed efficiency, reproductive performance, and subsequent impacts on progeny.

Procedure

Each year for three consecutive years, one hundred weaned Spring born Angus-crossbred heifer calves were purchased in early October from a local ranch near North Platte, NE and transported to the University of Nebraska, Lincoln West Central Research, Education, and Extension Center in North Platte, NE. Estrus was synchronized prior to artificial insemination (AI) in May. Following breeding, heifers were moved to upland range pastures. Pregnancy status was determined at the end of September via ultrasound (ReproScan, Winterset, IA).

Each year, forty AI bred heifers (134 ± 5 d post AI) were randomly selected and placed into a Calan Gate feeding system (American Calan, Northwood, NH). Heifers were acclimated to the gated feed bunks for 27 ± 4 d that limit access to a sole individual. In a 2×2 factorial design, heifers were assigned two treatments mixed with 4 oz chelated breeder mineral provided as top dress: with or without 200 mg/d monensin (Rumensin; Elanco, Greenfield, IN; RUM or CON, respectively) and with or without 1 lb/d dried distillers grains (DDG or HAY, respectively). The four resulting treatment combinations were CON:HAY, RUM:HAY, CON:DDG, and RUM:DDG. 

Treatment assignments were blocked by body weight (BW) and previous average daily gain (ADG) across four pens and within each pen. During the 88 ± 5-d treatment period, which ended 249 ± 5 d post AI, all heifers were fed ad libitum ground hay mixed with water delivered twice daily. Hay consisted of a mixture of prairie hay and alfalfa. Treatments were administered once daily as a top dress. Hay intake was calculated by weighing the hay delivered twice daily and subtracting orts collected each week. Hay and dried distillers’ grain were analyzed annually (Ward Laboratory Inc, Kearney, NE; Table 1).

Calving date ranged from February 17th to March 25th. Calves were weighed within 24 h of birth. An experienced technician scored calving difficulty (0 = no assistance, 1 = needed assistance) and calf vigor (1 = nursed immediately, 0 = did not nurse immediately). Each breeding season, bulls were introduced 5 d before prostaglandin administration in late May for a 60-d breeding period. Cow-calf pairs grazed pasture until weaning in late September, at which time cows and calves were weighed. Weaned calves were transitioned to a high concentrate feedlot ration over a 14-d acclimation period (Table 1). Calves from Year 1 entered the GrowSafe feeding system (GrowSafe Systems Ltd., Calgary, AB, Canada) for 118 d, which monitored intake. Calves from Year 1 and 2 (Year 3 is incomplete) were implanted with Synovex Choice and Synovex Plus (Zoetis Services LLC, Parsippany-Troy Hills, NJ) during the feedlot phase. Body weights were recorded throughout the feeding period until harvest at Tyson Foods (Lexington, NE). Final live body weight was calculated from hot carcass weight and assumed a 63% dressing percentage. Carcass characteristics were also recorded. 

Data were analyzed using PROC GLIMMIX of SAS 9.4 (SAS Inst. Inc., Cary, NC), with individual heifer serving as the experimental unit. Fixed effects included monensin supplementation (with or without) and dried distillers grains (with or without). Repeated measures of BW were analyzed by month of age. Interactions between fixed effects were not significant for cows or calves. Year was included as a random effect in the model and calf sex was included as a random effect when analyzing calf responses. Data were considered significant at P ≤ 0.05 before rounding and a tendency if P ≤ 0.10 and P > 0.05.

Results

Heifer Performance

Table 2 contains observed effects of treatment on heifers, but results do not include any data from after breeding in Year 3. Treatment interaction did not significantly affect BW (P = 0.90). Heifer BW did not significantly differ between treatments before treatment began (P ≥ 0.14). After treatment ended, heifer BW was 20 ± 9 lbs greater in DDG compared to HAY heifers (P = 0.04), but not significantly different in RUM compared to CON heifers (P = 0.18). Dried distillers’ grains provided greater nutrition than hay alone, and this led to greater BW. Heifer BW was not significantly different between treatments (P ≥ 0.18) at any other time point. 

Treatment interactions for ADG were not significantly different (P ≥ 0.09). Average daily gain during the treatment period was significantly greater in DDG compared to HAY heifers (P < 0.01), which is reflected in the 20-pound BW difference. Between calving and breeding, body weight change in HAY heifers was 0.2 lb/d greater than DDG heifers (P = 0.05). This suggests HAY heifers did not have as much BW to lose after treatment ended, and the nutritional needs of lactation began. Between breeding and weaning DDG heifers tended to have greater ADG than HAY heifers (P = 0.07). No other significant interactions between treatments exist (P ≥ 0.13). Hay intake during treatment was not significantly different between treatments (P ≥ 0.66). Due to small differences in ADG during treatment, the feed to gain ratio was significantly greater in CON heifers than RUM heifers (P = 0.04) and in HAY heifers than DDG heifers (P < 0.01). Greater available nutrients from dried distiller’s grains and possible changes in gut microbiome from monensin potentially improved hay feed efficiency. 

Gestation length was not significantly different by treatment (P ≥ 0.49). First calving difficulty was significantly greater in HAY (16%) compared to DDG cows (4%, P = 0.05). Calf vigor was not significantly different between treatment groups (P ≥ 0.12). Pregnancy to natural service as a primiparous cow was not significantly different (P ≥ 0.42). During the second parturition, calf vigor was not significantly different between treatments (P ≥ 0.17), and calving difficulty was 0% for all treatments. Julian birth date of second calves did not significantly differ (P ≥ 31), though second calf birth weights were heavier in RUM supplement cows compared to CON cows (P = 0.04). Sire effects may have influenced second calving results; however, parentage was not recorded. 

The cost of DDG during the treatment period was about $10/hd and that temporarily increased BW by 20 lb while decreasing calving difficulty. The cost of Monensin during treatment (0.19 lb/hd) was about $2.75/hd and that decreased the hay required for every pound of BW gained by 1.7 ± 0.9 lb/hd. 

Calf Performance

No significant interactions between treatments were observed for calf performance (Table 3; P ≥ 0.10). Calf body weight was significantly greater in CON offspring compared to RUM offspring at weaning (P = 0.03); however, no differences were observed at any other time points (P ≥ 0.11). Calf ADG during any time period between birth and harvest was not significantly different between treatments (P ≥ 0.20). 

There were no significant differences in feedlot intake (P ≥ 0.24) or feed efficiency (P ≥ 0.33) during Year 1 of the study. After two years of calf data collection, marbling scores tended to be greater in offspring from DDG supplemented dams compared to those from HAY supplemented dams (P = 0.10). Backfat tended to be greater in CON offspring than RUM offspring (P = 0.09). No other significant differences in carcass characteristics were observed (P ≥ 0.19). Collection of a third year of offspring data may provide a clearer understanding of the potential long-term effects of late gestation maternal supplementation. 

Conclusions

This ongoing study suggests that small differences in supplementation during late gestation have meaningful impacts on performance. Supplementing growing heifers with RUP temporarily increase BW, decreases calving difficulty, and may have increased offspring marbling at slaughter. Supplementing growing heifers with Monensin increased hay feed efficiency and may have decreased offspring backfat at slaughter. More observations are needed to confirm these trends. 

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