Cotton by-products have offered an economical source of protein to cattle for nearly 100 years. However, accompanying the relatively low price is a well-known risk due to the component gossypol. Ingested in large amounts, this toxic polyphenolic pigment can compromise liver functions, respiration rate, erythrocyte oxygen carrying or releasing capacity and reproductive capacities. In extreme cases, gossypol toxicity has proved fatal.
Concern about gossypol naturally increases when other protein sources, such as soybeans, become higher priced and least-cost ration formulation encourages the greater use of cottonseed products. Fortunately, recent research at the University of Florida (Velasquez-Pereira et al., 1996) has found that incorporating supplemental vitamin E in the diet can effectively reduce or avoid gossypol toxicity even at very high levels of cottonseed use. In fact, the cattle that were fed cottonseed meal and also vitamin E supplementation in this study performed as well as those fed soybean meal.
The results in this study are in keeping with earlier research that showed a proliferation of radical oxygen molecules and resulting depletion of cellular antioxidants with gossypol toxicity. Bender et al. (1988) found that plasma levels of vitamin E, ascorbic acid and glutathione peroxidase were all depressed in rats fed high levels of gossypol. Similarly, Lane and Stuart (1990) found that dairy cattle receiving high levels of gossypol also had lower plasma vitamin E levels than those not receiving gossypol.
Yet what constitutes a toxic level of gossypol consumption still needs to be determined. Dr. Lee McDowell of the University of Florida notes that mature cattle seem to have a large capacity to detoxify gossypol in the rumen, and some research suggests it would be unlikely for toxicity to occur even if mature animals' diet contained up to 25 percent whole cottonseed.
Typically, whole cottonseed has a gossypol content of 0.9 to 1.2 percent, so cattle receiving 7.3 lbs (3.3 kg) of whole cottonseed would consume 30 to 40 g of free gossypol. However, McDowell also notes that problems have been reported when cattle consumed only 4 to 5 g of free gossypol per head daily. The source of the gossypol fed can also affect the risk of toxicity. For example, the gossypol in cottonseed meal is more readily absorbed than that in whole cottonseed.
Research results with young bulls and heifers are generally more dramatic than with mature cattle, especially when the effects of gossypol on reproductive performance are measured. McDowell et al. (1996) noted that various studies have shown that gossypol toxicity delays puberty, impedes sperm motility, inhibits embryo implantation and delays embryo development. Chase et al. (1989) reported delayed puberty in bulls fed 27.3 mg of free gossypol per pound of body weight (60 mg/kg body weight) but found no effect at 2.7 mg per pound of body weight (6 mg/kg).
In the recent Florida study assessing the benefits of vitamin E supplementation when feeding cottonseed products, Velasquez-Pereira et al. (1996) formulated diets to contain 6.4 mg of free gossypol per pound of body weight (14 mg/kg). Thus, total daily intake ranged from 2.3 g at six months to 5.3 g at 16 months. The 24 bulls in the study were divided by three treatments:
- Soybean meal (control)
- Cottonseed meal
- Cottonseed meal and 4,000 IU of vitamin E per head daily.
The bulls receiving cottonseed meal without vitamin E supplementation showed signs of gossypol toxicity through delayed sexual development. In fact, by 16 months of age, four of these bulls still had not reached puberty (based on total sperm count and sperm motility), whereas all of those in the two other treatments had reached it (Figure 1). The bulls receiving unsupplemented cottonseed meal also had the lowest body weight gains of the three treatments.