Added fat has often been used in the diet of cattle to increase the energy density of the ration, because fat has a higher number of calories per unit of weight than carbohydrates and sugars. This was particularly important for feedlot animals where above-average growth rates were expected, and for dairy cows with high levels of milk production.
Excessive use of fat in the diet (more than 7 percent of the dry matter intake) can be detrimental, because it may be unpalatable and cause the loss of rumen microbes. There is considerable evidence (Schneider et al, 1988; Sklan et al, 1989; Ferguson et al, 1990) suggesting that the addition of limited fat supplements - particularly polyunsaturated fats from plant sources - to the diet of beef cows prior to breeding will enhance reproduction by reducing the anestrus period after calving.
There are several sources of fat available for use in cattle diets, including commercially prepared fat additives such as Megalac. Other sources include fish meal, oilseeds such as whole soybeans and whole cottonseeds, and food byproducts such as bakery wastes. The latter three are often available in this region.
How does fat contribute to reproductive performance? Several studies have shown that added fat will increase the number of follicles that will be formed on the ovary of the cow. However, added fat will not influence the number of follicles that are present in cows that have been "superovulated" and used in embryo transfers.
In addition, fat will increase the efficiency of rumen fermentation in producing energy for the cow; plant-derived oils (PUFA) have been shown to increase insulin and growth hormone levels; and luteinizing hormone secretion can be increased with fat supplementation. In Brahman x Hereford cattle, ovarian follicular growth was stimulated more by fat as compared to equal energy from carbohydrates, with a greater effect observed for fats with higher PUFA (Thomas et al, 1997).
Currently, research is inconclusive on exactly how to supplement fat in order to take advantage of any special qualities that lipids may have to improve reproductive performance beyond the energy contribution (Burns and Filley, 2010). Bellows et al (2001) observed that cow body condition and the quality and availability of forage will influence the result of added fat in the diet for reproduction.
The optimum level of supplementation has not been determined. However, a Missouri study has shown that feeding 2 pounds of raw cottonseeds per day for 60 days beginning 30 days before breeding season started increased the number of cows cycling at the beginning of the breeding season by 18 percent. Using this level as a guide, it remains to examine the economics of this supplementation for Pennsylvania and the mid-Atlantic region.
Whole soybeans can be used as the example, since they will be more readily available in this region. When priced at $9 per bushel, this results in a total cost of $180 to feed 10 cows for 60 days at 2 pounds per day. What returns can we expect from this supplementation?
First, the beans are an excellent source of energy and protein for the cows, and a conservative estimate (based on a hay price of $80 per ton) results in a substitution value of about $50 worth of hay.
Secondly, cows bred earlier in the breeding season result in calves being older and heavier at weaning the next year. If we actually wean nine calves from the 10 cows, we would need to increase weaning weight by about 6 pounds per calf to recover the rest of the cost of feeding the beans. This would be an average of about four to five days older when sold at weaning because of the supplementation.
Finally, as an energy and protein supplement, young cows can be expected to improve body condition and rebreeding rate, and cows with a higher potential for milk production may be able to produce more milk.
Cautions for the use of raw soybeans in beef cattle diets include (Lane, 2000):
- Raw soybeans contain urease, so they should not be mixed with urea in a ration.
- Storage molds can be extremely toxic, so beans with storage molds should not be fed to livestock unless extreme care is exercised.
- Beans should be below 13 percent moisture to prevent bin molding.
- Beans should be introduced to animals slowly and increased gradually.
- The beans should not be fed to young calves that do not have a functional rumen.
It will be important to watch the cost of the supplement, to optimize the amount that is fed, and to remember there are a lot of issues related to reproduction - general nutrition, herd health, bull fertility, weather and several others. While it's not a silver bullet that will replace good overall management, the evidence suggests supplementation with fat for the breeding herd may have some economic benefits.
Dr. John Comerford is an associate professor and extension beef specialist at Pennsylvania State University.