Heifer Study: University of Missouri researchers recently compared the use of MGA and controlled internal drug release (CIDR) inserts and their impacts on heifer estrous response, timing of artificial insemination (AI) and pregnancy rates.

A total of 353 heifers at three locations were randomly assigned to one of two treatments by age and weight.

  • 175 MGA Select-treated heifers were fed MGA for 14 days. Researchers administered GnRH 12 days after MGA withdrawal, and prostaglandin (PG) seven days after GnRH.

  • 177 head were inserted with CIDRs for 14 days. GnRH was injected nine days after CIDR removal, and PG was administered seven days after GnRH.

Heifers were monitored for heat beginning the day PG was administered. Researchers AI'd heifers 12 hours after the onset of estrus, and determined pregnancy rates by ultrasound 40 days later.

Researchers found:

  • Estrous response didn't differ between treatments.

  • Peak AI occurred on Day 3 for heifers in both treatments, but distribution of AI was more highly synchronized among CIDR- than MGA-treated heifers.

  • AI pregnancy rate was greater in CIDR-treated heifers (63%) than MGA-treated heifers (47%).

  • Final pregnancy rates did not differ between treatments.

“Replacing MGA with CIDR inserts improved synchrony of estrus and pregnancy rate resulting from AI in replacement beef heifers,” says the University of Missouri's (UM) Dave Patterson.

Cow Study: UM researchers evaluated 650 crossbred, lactating beef cows at four locations. Cows were assigned to groups based on age, days since calving and body condition scores (BCS).

  • Cows assigned to the MGA Select treatment (MGA Select; n = 327) were fed MGA for 14 days. GnRH was injected on Day 26, and PG on Day 33.

  • CO-Synch + CIDR-treated cows (CO-Synch + CIDR; n = 323) were injected with GnRH and fitted with an Eazi-Breed CIDR insert for seven days; PG was injected and CIDR removed seven days later.

AI was performed at 72 hours after PG for cows assigned to the MGA Select treatment, and at 66 hours after PG administration for the CO-Synch + CIDR treatment. All cows were injected with GnRH at time of AI.

Three AI sires were used at location 1, and one at locations 2, 3 and 4. Cows were exposed to fertile bulls for natural service 14 days after AI for a 60-day natural service period at locations 1, 3 and 4, and for a 45-day natural service period at location 2.

Researchers found:

  • No differences between treatments at the respective locations for age, days postpartum, BCS or estrous cyclicity status at the initiation of treatment. However, there were differences among locations.

  • There was no effect of treatment, technician, or sire on pregnancy rates resulting from fixed-time AI.

  • Pre-treatment estrous cyclicity before the initiation of the MGA Select or CO-Synch + CIDR protocols didn't affect pregnancy rates from fixed-time AI.

  • Final pregnancy rates did not differ between treatments.

“These results indicate synchronization with the MGA Select and CO-Synch + CIDR protocols produce comparable pregnancy rates to fixed-time AI when inseminations are performed at 72 and 66 hours after PG, respectively,” Patterson says. “The results provide producers a choice and means for expediting genetic improvement and reproductive management.”

This article was provided by the National Association of Animal Breeders. Learn more at www.NAAB-CSS.org or call 573/445-4406.

Table 1. Estrous response, AI pregnancy and final pregnancy rates
Estrous response AI pregnancy rate Final pregnancy rate
CIDR 154/177 (87%) 112/177 (63%)a 164/177 (93%)
MGA 147/175 (84%) 83/175 (47%)b 159/175 (91%)
Total 301/352 (86%) 195/352 (55%) 323/352 (92%)
Difference +3% a,bP = 0.01 +16% +2%
Source: From Kojima et al. (2004)
Table 2. Pregnancy rates after fixed-time artificial insemination and at the end of the breeding season
Pregnancy rate to fixed-time AIa Pregnancy rate at end of breeding seasonb
Item Proportion % Proportion %
Location 1
MGA Select 70/106 66 99/106 93
CO-Synch + CIDR 67/104 64 99/104 95
Location 2
MGA Select 53/80 66 77/80 96c
CO-Synch + CIDR 56/78 72 76/78 97c
Location 3
MGA Select 26/45 58 42/45 93
CO-Synch + CIDR 29/43 67 42/43 98
Location 4
MGA Select 52/96 54 87/96 91
CO-Synch + CIDR 62/98 63 91/98 93
MGA Select 201/327 61 305/327 93
CO-Synch + CIDR 214/323 66 308/323 95
a. Pregnancy rate to fixed-time AI determined by ultrasound 40 to 45 days after AI.
b. Pregnancy rate at the end of the breeding season determined 50 to 60 days after the end of breeding season.
c. Pregnancy rate at the after-45-days breeding season.
Source: From Schafer (2005)