Over the past few years it has become common for producers to feed at least some fat in cattle rations and supplementation programs. Because of university and industry work in this area, the effect of fat status in cattle is well documented. While feeding fat is well known to stimulate gains in finishing cattle recent research into the effects of fat inclusion in the diet on reproduction has shown positive results.
In the mid 1980¡¦s work was done comparing the reproductive status and return to cycling of cows in optimal condition and cows in poor condition receiving supplementation with whole cottonseed (which will run 18 to 22 percent fat on a dry matter basis). The reproductive performance receiving the whole seed supplementation was similar to that of the cows in better body condition. More recent studies have compared various fat types from different plant and animal sources as well as investigated the actual physiological changes that take place when fat is used as an energy supplement.
It would be useful to take a look at some of the work that has been done in this area to reinforce the concept of fat feeding and supplementation in beef cattle. This article will be somewhat more technical than normal but will provide some insight into this area of beef cattle nutrition.
Research into Fat Applications in Cattle Feeds
In 1999 Williams and Stanko with Texas A&M University in Beeville evaluated dietary fats as reproductive ¡§nutraceuticals¡¨ in beef cattle. Nutraceutical is a current buzz word used to describe a dietary or nutritional product that has a therapeutic effect on the animal to which it is fed. Essentially it acts like a naturally occurring health-stimulating product. They pointed out that inadequate dietary energy intake and poor body condition are two of the most pervasive factors influencing reproductive efficiency in beef cattle production systems. This being said it is an important goal to discover novel methods for enhancing reproductive processes in cattle that are exposed to these conditions, often on a repeated basis and in many cases related to environmental effects. Their review examined dietary fat as a reproductive "nutraceutical," including the role of fatty acid content and minimum effective intake. Although the consumption of neutral lipids (fats) by ruminants is limited under natural conditions, the addition of digestible fats to increase energy (caloric) content or to positively modify diet physical characteristics is a long-standing practice. In recent history, fat supplements have been used in attempts to influence specific metabolic pathways and, ultimately, hormones that directly effect ovarian cellular processes. The basis for this approach lies within an array of digestive, metabolic, and reproductive changes that occur when cattle consume significant quantities of digestible fat. The evidence suggests that the consumption of fat by cattle, particularly poly-unsaturated plant or vegetable oils, can positively influence 1) ovarian follicular growth; 2) luteal function; and 3) postcalving reproductive performance independent of effects seen from energy supplementation. When we look at the mechanisms put into effect by the feeding of fat, we find that these effects have been attributed to a cascade of events that change rumen fermentation patterns, heighten lipoprotein-cholesterol synthesis, increase secretion of ovarian steroids, modify circulating concentrations of insulin and GH, and enhance the synthesis or accumulation of IGF-I in ovarian cells in addition to other chemical and physiological effects. Whole oilseeds, oil milling by-products, and some manufactured products have been found to produce these physiological effects.
Another study in 2000 by Mattos, Staples and Thatcher with the University of Florida further examined the effects of dietary fatty acids on reproduction in ruminants. As noted above, fats in the diet can influence reproduction positively by altering both ovarian follicle and corpus luteum function. This is a result of improved energy status and by increasing precursors for the synthesis of reproductive hormones such as steroids and prostaglandins. They examined the difference in the types of fatty acids fed. Remember that fats as a nutrient class are made up of different ¡§fatty acids which vary in length of the molecule chain and how the molecules are put together. This study found that certain fatty acids (particularly those from the n-3 family) reduce ovarian and uterine synthesis of PGF2, decrease ovulation rate in rats and delay the birth process in sheep and humans. Subsequently they found that the manipulation of the fatty acid profile of the diet (by changing the fat sources and the amounts of each fed) can be used potentially to amplify suppression of uterine synthesis of PGF2Ñ during early pregnancy in cattle, which may help reduce embryonic mortality. Feeding fats and targeting of fatty acids to reproductive tissues may be a strategy to combine nutrition and reproductive management to improve animal productivity.
In still another 1997 study Thomas, Bao and Williams evaluated how dietary fats varying in their fatty acid composition might influence follicular growth in cows fed rations that were different in composition but the same in energy level. In other words, they varied the types and levels of fatty acids in the diet but not the overall energy, protein and fiber content. In this study they looked at rations that contained no added fat (Control, CT), diets supplemented with fats containing primarily saturated fats (SAT), polyunsaturated fats (PU), or highly polyunsaturated fats (HPU). Initially they found that body weights and body condition scores remained similar for all groups throughout the study. Polyunsaturated fat (PU) increased the number of medium-sized follicles on days 5 through 9 of a synchronized estrous cycle within 3 weeks of onset of feeding and maximized this to a fourfold difference after seven weeks Fats with predominantly SAT and HPU tended to produce these effects after seven weeks, a later effect. All fat-supplemented diets increased serum concentrations of total cholesterol, growth hormone and follicular fluid IGF-I in large follicles compared to CT. Overall they concluded that consumption of PU fatty acids stimulates a greater rate of ovarian follicular growth in cattle compared to CT, AT, and HPU.
Work has also been performed looking at the effects of fat supplementation on different breeds. De Fries, Neuendorff and Randall (1998) took a look at fat supplementation and how it influences breedback in Brahman cows. Initially they took several older Brahman cows in excellent body condition (6.5+) and assigned them to one of two treatments. These were rations including 5.2 percent fat from added rice bran or 3.7 percent fat which was the control diet. As in the study above they were designed to be of the same energy and protein content. All cows were exposed to a fertile bull at the estrus following the first normal estrous cycle and for a 60-day breeding season. Cows receiving rice bran gained more body condition than cows receiving the control supplement. The numbers of small (< 4.0 mm), medium (4.0 to 7.9 mm) and total follicles were greater in the rice bran than in the control group from 15 to 29 day after calving. Fat supplementation increased the numbers of medium, large and total follicles and size of the largest follicle during the third week before the first normal estrous cycle. The intervals from parturition to reproductively important end points were similar between dietary treatments as well as the percentage of cows showing normal or abnormal estrous cyclic activity. Treatment did not affect daily serum progesterone (P4) concentrations, however, there was a tendency for more rice bran-supplemented cows to be pregnant (94.1 vs 71.4 percent). Calf weight gain tended to be higher in calves nursing rice bran-supplemented dams. They concluded that using rice bran, which normally carries higher concentrations of oleic and linoleic acids, as a fat supplement for postpartum cows enhanced ovarian follicular growth before normal estrous cycles resumed and increased body condition scores and pregnancy rates without altering time to rebreeding or serum progesterone concentrations.
Research has also found a nutritional effect of feeding different fat sources to heifers. Filley and co-workers at Oregon State University (2000) examined the effect of rumen by-pass fats to first-calf Hereford X Angus heifers to determine if feeding rumen-protected fatty acids (FA) increase plasma concentrations of linoleic acid and PGF2 as well as other blood metabolites. Interestingly and in contrast to other studies shown, while supplemental fat sources fed to first calf beef heifers increased plasma levels of linoleic acid and production of PGFM in the early postpartum period, it did not improve the overall fertility of these heifers in the subsequent breeding season.
In yet another heifer study Lammoglia, et al (2000) considered the effects of dietary fat and sire breed on puberty, weight, and reproductive traits of F1 beef heifers. This project compared prepubertal F1 heifers from crossbred dams bred to either Hereford (H), Limousin (L), or Piedmontese (P) sires which were fed 1.9 percent (Low fat, LF) or 4.4 percent (High Fat, HF) dietary fat from about 254 days of age until they reached puberty or the breeding season started. In this case they used safflower seeds (37 percent oil with 79 percent linoleic acid) were the added fat source. They found that while gain, average daily gain, body condition score, and backfat thickness were affected by sire breed it was not effected by the diet. The supplementation program did not affect pubertal age but tended to affect the percentage of heifers pubertal by the beginning of breeding. As expected age at puberty as well as percentage pubertal at the beginning of breeding was highly affected by sire breed effects. No significant diet effects were noted in the overall study and the workers concluded that effects of supplemental dietary fat may be breed-dependent and they hypothesized that a feeding period of approximately 60 days in duration may be more appropriate than the 162 days used in this study.
Various studies have shown similar positive responses. There are several hypotheses currently existing for why these effects are observed. These include:
1) The feeding of the additional energy in the form of fat improves the cow¡¦s overall energy status so that she returns to estrus sooner after calving and therefore conceives earlier.
2) Cows fed fat secrete more progesterone, a hormone necessary for implantation and nutrition of the newly formed embryo. One of the effects seen from the feeding of fat is an increase in the size of follicles produced on the ovaries. A review of a series of studies investigating this concept showed that follicle size increased an average of 27.13 percent. The increase in follicle size also resulted in an increase in corpus luteum (CL) size. The CL is the structure which produces progesterone.
3) Specific individual fatty acids (components of fats) inhibit the production of PGF2Ñ by the uterus. This prevents the regression of the (CL) on the ovary so that the newly formed embryo survives. In other words, the CL has a longer lifespan which increased the length of the estrus cycle. This means that a cow has a longer period of time in which to be bred when she is in heat. Certain fat types have been shown to be especially effective in creating this result. Fats higher in linoleic acid (a specific long-chain fatty acid) have been shown to be especially useful in this area
4) Higher dietary fat levels may increase the concentration of cholesterol in the blood stream which acts as precursors or building blocks for reproductive hormones such as progesterone. Workers noted differing effects in different breeds and between the use of fat supplementation strategies between cows and heifers.
Regardless of the exact mechanism, the use of fat in supplementation of brood cows has proven very effective in improving reproductive performance. More research is needed to identify specific feeds or grains that possess the necessary fatty acid profile to produce an optimal response (or at least the best improvement). Cows should receive supplements higher in fats just prior to calving and in the early postpartum period to enhance this effect. Feeding a higher fat supplement which results in the cow receiving 3 to 5 percent fat in her overall daily diet should improve reproductive rates in most cases. Be sure not to exceed 6.5 to 7 percent fat in the daily diet as this can result in reduced fiber digestion in the rumen.
Dr. Steve Blezinger is and nutritional and management consultant with an office in Sulphur Springs Texas. He can be reached at Route 4 Box 89 Sulphur Springs, TX 75482, by phone at (903) 885-7992 or by e-mail at firstname.lastname@example.org.