Meeting nutrient requirements of the replacement heifer and the pregnant or lactating female is of critical importance in assuring peak reproductive performance. These requirements include protein, energy, minerals, vitamins, dry matter, and water. Research has indicated adequate fat may be an additional nutrient that needs to be present in the diet. Research has evaluated the effects of supplementing the gestation diet of first-calf heifers with fat supplied from various oil seeds. Two studies were conducted on pregnant first-calf heifers.

Study 1

In the first study, dams were fed diets containing 1.7% fat for the control diet and 4.7% fat in the experimental diet, with both diets being equal in protein and energy content. Fat content of the experimental diet was increased by adding 3 lb of safflower seeds containing approximately 37% oil. Seeds were run through a roller mill with enough pressure to crack approximately 90% of the hulls while not causing oil loss. This process increased digestibility by over 44%. The diets were fed for an average of the last 53 days before calving and supplemental feeding was stopped at calving when the dam-calf pairs were placed on range pastures. At 5 hours of age, the newborn calf was placed in a cold room maintained at 32°F and remained there for 140 minutes. Rectal temperatures and blood samples were taken every 10 or 20 minutes throughout the cold exposure period. Results show that calves from dams that received supplemental fat during gestation responded to the cold stress better than did calves from dams that did not receive the added fat (Figure 1 and Figure 2). These calves had a greater increase in body temperature and were able to maintain the increased temperature throughout the cold exposure. In addition, these calves had greater blood glucose concentrations. This is important as it indicates the calves had more “fuel” available for producing heat through shivering thermogenesis (heat production).

Study 2

The procedures for the second study were similar to Study 1 with diets fed for an average of 55 days before calving and supplement feeding was stopped at calving. Diets were equal in energy and protein content and contained 2.2% and 5.1% fat for the control and high-fat diet, respectively. Fat content of the experimental diet was increased by adding 5 lb of safflower seeds. Calves were placed in the cold room five hours after birth with temperature and blood sampling procedures the same as in Study 1. Results are summarized in Figure 3 and Figure 4 and show essentially the same calf response as obtained in Study 1. The two studies indicate calf response to cold temperatures immediately after calving was improved by feeding the dam supplemental fat during gestation.

Pregnancy Rate
Additional data were obtained in Studies 1 and 2. Results are summarized in Table 1. Calf birth weights, calving difficulty scores, and calf vigor scores were not affected by feeding the high-fat diets. However, pregnancy rates were greater in dams that received fat supplementation during gestation. This increase in pregnancy rate is of special interest since fat supplementation ceased at calving. This suggests a carry over effect of supplemental fat fed during gestation.

Heifers
Our work has continued on effects of feeding fat during gestation on subsequent reproduction in heifers. These studies have included safflower seeds, raw soybeans, and sunflower seeds as fat sources. Oil seeds were run through a roller mill to crack the hulls, but without oil loss. Diets were formulated to contain approximately equal amounts of protein and energy and contained 2.0, 4.2, 3.3, or 4.5% fat for the control, safflower, soybean, and sunflower diets, respectively. Diets were group fed in dry lot for an average of 65 days precalving with supplemental fat feeding ending at calving. Estrus was synchronized with two injections of prostaglandin and bred in a 35-37 day breeding period. Results are summarized in Table 2.

There did not appear to be differences among the three fat sources so the comparisons discussed will be for the average of the diets containing supplemental fat vs. the control diet. Feeding fat resulted in a non-significant increase in calf birth weight and in the incidence and severity of calving difficulty. But, the interesting increase found was a 13 percentage point increase in fall pregnancy rate and a 30 lb increase in calf weaning weight.

A study on effects of feeding fat to replacement heifers was also conducted. The study involved 246 weaned, crossbred heifers sired by Hereford, Limousin, or Piedmontese sires that were held in feedlots and fed diets containing either 1.9% or 4.4% fat from 254 days of age until they reached puberty or until the breeding season started. The feeding period averaged 162 days. Fat content of the diet was increased by adding 1.5 lb safflower seeds to the daily ration. At the beginning of the breeding season, heifers were moved from the feedlots to irrigated pasture and supplemental feeding was stopped. Heifers were bred in a 54 day AI breeding season. Results are summarized in Table 3. Diet effects on percent puberal at the beginning of the breeding season tended to be affected, but diet effects on age at puberty in heifers that reached puberty by the beginning of breeding or on final pregnancy rate were not different.

Sire effects were important for all three endpoints with the general pattern being that Limousin-sired were the oldest with fewest puberal at the beginning of the breeding season, Piedmontese the youngest and highest percentage puberal at begin breeding, and Herefords were intermediate.

Some interesting effects of fat feeding on the different breeds represented in the study were noted. Statistically, this is called a diet x breed interaction and was significant. Feeding fat had little effect on the percentage of heifers puberal in the Hereford-sired group (74 vs. 76%), reduced the percentage in Limousin-sired (70 vs. 61%), but increased the percentage in the Piedmontese-sired group (76 vs. 98%) (Table 3). Numerous studies have indicated Piedmontese is a double-muscled, low-fat breed. Does this interaction suggest the dietary fat “requirement” for breeds of lower body fat content may be different than those for breeds with greater body fat content? If this is true, it has interesting implications as we consider present selection pressure in the industry being for animals with decreased body fat. We think the 162 day fat-feeding period may have been too long for the Hereford and Limousin breeds. The results may have been more favorable if the feeding period would have been shorter.

Many studies have been conducted or are underway to determine possible mechanisms acting in the effect of fat on reproductive activity. Animal scientists in Florida have suggested that some of the fatty acids in the diet may inhibit synthesis of prostaglandin by the uterus resulting in extended life of the corpus luteum and its production of progesterone, thus avoiding death of the early embryo.

Summary
Results of research indicate supplemental dietary fat can affect reproductive end points in cattle. Results of our work suggest a positive response in pregnancy rate and calf weaning weights in lactating heifers when supplemental fat was fed for the last 50-60 days of gestation. The mechanisms of action are not clear at this time.

†Since authoring this article, Dr. Bellows has retired from the USDA. This research was conducted under a cooperative agreement between USDA-ARS and the Montana Agric. Exp. Sta. Mention of a proprietary product does not constitute a guarantee or warranty of the product by USDA, Montana Agric Exp. Sta., or the authors and does not imply its approval to the exclusion of other products that may be also suitable. USDA, Agricultural Research Service, Northern Plains Area, is an equal opportunity/affirmative action employer and all agency services are available without discrimination.