Having cows in good body condition at calving is essential to a successful breeding season. However, nutrition, animal management and specific nutrient supplementation during the early portion of the breeding season can alter pregnancy rates.
Impact of body condition score (BCS) at calving
Energy reserves at the time of calving affect the ability of cows to become pregnant in the next breeding season.
Cows in BCS 5 (1 = emaciated to 9 = obese) or better at calving have fewer days to first estrus and increased pregnancy rates. Cows calving in BCS 4 or less have a 9% to 29% lower pregnancy rate compared to cows calving at BCS 5 or greater.
Pregnancy rates for cows of various BCS are illustrated in Figure 1.
Research indicates that changes in BCS between 4 and 6 have a greater impact on pregnancy rate than changes in BCS above 6 or below 4 (see dotted lines).
In addition to reduced pregnancy rates, cows calving at BCS 4 or less become pregnant later in the breeding season (Table 1).
First-calf heifers are even more sensitive to the effects of BCS at calving on pregnancy rates. Dramatic decreases of 40% to 50% occur as heifers drop from BCS 6 to BCS 4. In contrast to mature cows, heifers exhibit a significant decrease of approximately 16% in pregnancy rate between BCS 6 and BCS 5. Therefore, the optimum BCS at calving is 6 or 7 in heifers.
Nutrition during the breeding season
On most ranches, the breeding season coincides with some of the best forage of the year. In general, little thought is given to nutrition during the breeding season except for mineral supplementation. However, nutrition during the early portion of the breeding season may impact embryonic mortality and the percentage of cows conceiving early in the breeding season.
Early research in breeding season nutrition focused on trying to make up for poor body condition at calving. Thin cows gaining weight during the breeding season have greater pregnancy rates than thin cows that don’t gain weight. However, increasing energy and protein intake post-calving to thin cows still does not produce pregnancy rates comparable to cows that calve in good body condition. Thin cows are delayed in cycling after calving.
Recently, research has focused on nutrition around the time of artificial insemination (A.I.) or the beginning of the breeding season. The period from insemination or breeding until maternal recognition of pregnancy on day 15 to 17 after breeding is critical. Attachment and adhesion of the embryo to the uterus occurs between day 19 and 24 after breeding. Early embryonic loss occurs during the first 28 days after breeding. So management or nutritional strategies during the first 21 to 30 days of the breeding season may reduce early embryonic mortality and improve pregnancy rates.
South Dakota researchers examined the effect of a rapid change in nutrition just after breeding. In Northern climates, heifers may be developed in drylot until the start of the breeding season. When heifers are turned out to pasture or range immediately after A.I. or at the beginning of the breeding season, they experience a drastic change in nutrition and grazing behavior. As a result, heifers lose weight and have reduced early season pregnancy rates compared to heifers developed from weaning to breeding on pasture (Table 2).
Moving heifers from drylot to pasture resulted in a 12% decrease in A.I. pregnancy rates, but final pregnancy rates were not different between the two groups – therefore, changing diet and environment resulted in a “pasture crash” for drylot heifers. This “pasture crash” resulted in fewer heifers conceiving early in the breeding season, probably due to greater early embryonic mortality. To alleviate the detrimental effects of the change in diet and environment:
- Develop heifers on pasture or range from weaning through breeding.
- Supplement heifers moved from drylot to pasture after A.I. with energy and protein such as dry distillers grains.
- Develop heifers in drylot but move heifers to pasture 40 to 45 days before A.I. to adapt them to the nutritional and environmental change.
Is there any nutritional supplementation that will improve early breeding season pregnancy rates? Healthy rapidly growing embryos produce more of the proteins needed for maternal recognition of pregnancy compared to embryos that are delayed in growth. Essential fatty acids are needed by the developing embryo/conceptus for proper membrane development and enzyme function. Researchers from Texas A&M found that supplementation of essential fatty acids from A.I. to day 21 post-A.I. can increase conceptus size and produce greater concentrations of mRNA for important maternal recognition factors.
Calcium salts of soybean oil (CSSO) supply high levels of essential fatty acids that influence conceptus development. In a study using over 770 cows, Dr. Reinaldo Cook (Texas A&M University) and colleagues found that supplementing cows with 100 grams of CSSO from A.I. until 21 days after A.I. increased pregnancy rates over cows supplemented with a fat source that did not contain high levels of essential fatty acids (Table 3).
BCS and the percentage of cows showing heat before A.I. were not different between the two fat sources, indicating that the CSSO probably increased pregnancy by improving embryo/conceptus growth and maternal recognition factors.
The effects of CSSO on pregnancy rates were similar in Bos Taurus and Bos Indicus cattle, indicating CSSO supplementation will work across all cattle types. Most forms of CSSO are granular, but recently some companies are including CSSO in low-moisture molasses tubs. Recently, Texas A&M researchers compared the granular form of CSSO with CSSO in low-moisture tubs (Midcontinent Livestock Supplements). Whether CSSO came from the granular or tub form, the concentrations of essential fatty acids in circulation were increased to the same extent compared to unsupplemented cows.
As the breeding season approaches, keep cows in good body condition and consider adding management or supplementation strategies to increase early season pregnancies.
