Effect of Spring and Fall Calving on the Subsequent Impact of Monensin Supplementation in 1st and 2nd Parity Cows

A study was conducted to determine the effects of monensin; an ionophore that improves energy efficiency through alteration of VFA production in the rumen, in first or second parity lactating beef cows in a spring- and fall-calving herd. One hundred thirty-five (BW = 455 ± 20.2 kg) Angus crossbred cows and heifers (n = 88 fall calving; n = 47 spring calving) were blocked by calving season, stratified by parity, calving date and sire then randomly assigned to pastures within herd in a randomized complete block design. Pastures (n = 4/herd) were assigned randomly to 1 of 2 treatments: 1) CON (no supplemental monensin), 2) MON (200 mg/d monensin). All cattle received 2.3 kg/d of grain based supplement which served as the treatment carrier. Supplementation began 35 d prior to breeding in both herds and continued through weaning in the spring herd and through breeding in the fall herd. Body weights and BCS were collected at the initiation, 3 interim d, and end of study in both herds while hair coat scores (HCS; 1 = slick, short summer coat, 5 = full winter coat) were recorded for all cattle on the final d. Quantitative variables were analyzed using PROC MIXED and categorical variables were analyzed using PROC GLIMMIX with an alpha of 0.05 for all dependent variables. Final cow BW were similar between treatments and calving season (P ≥ 0.42). In the spring-calving herd, MON increased cow pre-breeding ADG compared to CON and both treatments in the fall-calving herd (treatment × calving season interaction, P = 0.03). Calf BW and ADG were not affected by MON supplementation nor by calving season (P ≥ 0.54). Cow BCS were not affected by treatment (P ≥ 0.18); however, pre- and post-breeding BCS were greater in the spring-calving herd compared to the fall-calving herd (P < 0.01). Cow HCS was not affected by MON supplementation (P ≥ 0.23), but as expected, the fall-calving herd had a greater HCS than the spring calving herd (P < 0.01). The total percentage of cows bred regardless of conception method were similar between treatments (P = 0.80); however, there was a tendency for a greater percentage of pregnant cows in the spring-calving herd compared to the fall-calving herd (P = 0.06). Monensin supplementation did not affect gains of fall calving cows consuming low quality forage but may affect spring calving cows consuming a greater plane of nutrition.