Maximizing Reproductive Efficiency

The benefits and challenges of estrous synchronization for natural service

BY JOHN B. HALL, PH.D. Extension Beef Specialist University of Idaho

Many cow-calf operations in Idaho calve in February and March. One reason for choosing this calving season is to allow bulls owned by the ranch to breed cows for at least one cycle before moving to range. In addition, having a majority of the cows calve in the first 30 days of the calving season is critical to profitability.

One challenge in herds employing only natural service mating is how to maximize the percentage of cows bred early in the breeding season. Estrous synchronization can be used with natural service. Several easy systems have been developed.

Challenges With Estrus Synchronization And Natural Service

Synchrony. Clearly, one of the important factors in synchronizing cows for natural service is to ensure that the synchrony of estrus is not so tight as to overwhelm the bull power. For cows exhibiting natural estrus cycles, the percentage of cows in heat on any given day will be few and it will take 20 to 22 days for all cows to come in heat. The goal of synchronization for natural service is to have a majority of cows in heat within 7 to 10 days not 1 or 2 days like estrus synchronization for AI.

Bull Power. When using synchronization with natural service, bulls should be required to service fewer cows per bull. Recommended cow to bull ratios for natural service are 30:1. However, for synchronized natural service the cow to bull ratio should be 15:1 to 25:1 depending on the age of the bull. Older mature bulls are preferred over yearling bulls for synchronized natural service. Older bulls have a greater serving capacity and can handle more cows per bull. In addition, older bulls are more experienced and are less likely to miss cows in heat or continue servicing a cow multiple times.

All bulls need to pass a breeding soundness exam. Bulls need to be in top physical condition with body condition score of 5 and used to exercise. During the synchronized period, bulls should be observed daily for signs of fatigue, injury or failure to service cows.

Systems For Synchronizing Estrus For Natural Service

The system used for synchronization will depend on cow age (heifer vs cow), cycling status, and amount of labor needed. Each system uses a different synchronization drug and has a different protocol.

1 Shot Prostaglandin (PGF) - Natural Service (NS)

This program only works on cycling cows or heifers. It involves a single shot of prostaglandin F2α (Lutalyse®, Insynch®, ProstMate ®) or it’s analog (Estrumate®, estroPLAN®) to all animals 5 days after introduction of bulls (Figure 1). Only cycling cows with a functional corpus luteum (CL) will be synchronized by this treatment. The injection of PGF will lyse the CL and bring females into heat.

Bulls will breed cows in heat during the first 5 days. Any cow bred during the first 5 days will not respond to the prostaglandin treatment as the CL is too immature, so these cows will remain pregnant. After the shot of prostaglandin, unbred cycling cows will come into estrus over a 5-to-7-day period. In research conducted in Nebraska, cows synchronized with the 1 Shot PGF-NS system had a 12% increase in calves born in the first 21 days of the calving season compared to unsynchronized cows (75% vs 63%, respectively).

14 Day MGA - NS (Heifers only)

Melengestrol acetate (MGA®; Zoetis or HeifermaX®; Elanco) is a synthetic oral progestin that acts like the naturally occurring hormone progesterone in heifers. Treatment with MGA keeps heifers from expressing estrus. In addition, it will initiate cycles in non-cycling animals. MGA is only labeled for use in heifers.

The MGA is fed at 0.5 mg/heifer/day of MGA mixed in 3 to 5 lbs. of grain. Some feed mills provide MGA in a range cube. Heifers need to be fed at the same time every day and there needs to be enough bunk space, so all heifers get their share of the product.

Heifers are fed MGA for 14 days (Figure 2). After feeding is stopped, heifers will come into heat over a 3-to-5-day period. This heat is an infertile heat and bulls should not be introduced at this time. In addition, synchrony may be too tight. Bulls are introduced to heifers 13 days after the last feeding of MGA. The advantage to this system is response is good and it is effective in initiating cycles in non-cycling heifers. The disadvantage is that the system takes almost a month to complete before breeding can begin. Researchers from Kentucky worked with 601 heifers in 12 herds. Using the 14 Day MGA-NS method resulted in 69% of the heifers pregnant to the synchronized estrus and 83% of heifer were pregnant within the first 30 days of the breeding season (Patterson et al., 1991).

7-day CIDR - NS (Heifers or cows)

The Eazi-Breed CIDR (CIDR; Zoetis) is a plastic device that is impregnated with progesterone that is inserted in the vagina of the cow. Treatment with a CIDR keeps cows or heifers from expressing estrus. In addition, it will initiate cycles in non-cycling animals.

The CIDR is inserted on Day 0 and removed 7 days later. One day after CIDR removal bulls are turned in (Figure 3). For example, CIDRs are inserted on Monday, and the following Monday they are removed. Bulls are turned in on Tuesday.

Since the CIDR is only in for 7 days, heats after CIDR removal occur over 7 to 10 days and heats are fertile. In a study with multiple herds, herds that used the 7-day CIDR system had more cows pregnant in the first 30 days and a higher pregnancy rate overall (Table 1.)

A modification of the 14 Day MGA system is to feed MGA for 7 days then turn bulls in the day after the last feeding of MGA. Synchrony is not as good with this method, but it will “jump start” some non-cycling heifers.

If you are interested in synchronizing for natural service, synchronizing heifers is a good way to start. It will give managers a good idea of what to expect and allow them to decide if they want to expand it to the cow herd.

Considering the genetic and financial investment in bulls, it is worth considering estrous synchronization for natural service. Ensuring that more cows are bred to desired bulls and increasing weaning weights of calves will improve profitability and herd quality.

If you are interested in estrous synchronization for natural service, feel free to contact me jbhall@uidaho.edu or 208-756-2749.