RESUMEN
Dietary fat is fed to increase energy intake and provide fatty acids (FA) to support milk fat production. Oilseeds contain unsaturated FA that increase the risk for biohydrogenation-induced milk fat depression, but FA in whole cottonseed (WCS) are expected to be slowly released in the rumen and thus have a lower risk for biohydrogenation-induced milk fat depression. Our hypothesis was that increasing dietary WCS would increase milk fat yield by providing additional dietary FA without induction of milk fat depression. Four primiparous and 8 multiparous lactating Holstein cows, 136 ± 35 and 127 ± 4 DIM, respectively, were arranged in a replicated 4 × 4 Latin square design with 21-d periods. Treatments were WCS provided at 0%, 3.4%, 6.8%, and 9.9% of dietary dry matter, and WCS was substituted for cottonseed hulls and soybean meal to maintain dietary fiber and protein. Treatment did not change milk yield. There was a treatment-by-parity interaction for milk fat percent and yield with a quadratic decreased in primiparous cows but no effect of WCS in multiparous cows. Cottonseed linearly increased milk fat trans-10 18:1 in primiparous cows but not in multiparous cows. Increasing WCS increased milk preformed (18C) FA yield and partially overcame the trans-10 18:1 inhibition of de novo FA synthesis in the primiparous cows. Apparent transfer of 18C FA from feed to milk decreased in all cows as WCS increased, but the magnitude of the change was greater in primiparous cows. Increasing WCS decreased total-tract apparent dry matter, organic matter, and neutral detergent fiber digestibility. There was no change in total FA digestibility. However, 18C FA digestibility tended to be decreased in both parities and 16C FA digestibility was quadratically increased in multiparous cows but not changed in primiparous cows. Total fecal flow of intact WCS increased as WCS level increased, but fecal flow of intact seeds as a percentage consumed was similar across treatments. Fecal flow of intact seeds was greater in multiparous cows (4.3% vs. 1.1% of consumed). Plasma concentrations of glucose, nonesterified FA, triglycerides, and insulin were not changed. However, plasma urea-N increased with increasing WCS. Plasma gossypol increased with WCS (0.08-1.15 µg/mL) but was well below expected toxic levels. In conclusion, WCS maintained milk and milk component yield when fed at up to 9.9% of the diet to multiparous cows without concerns of gossypol toxicity, but primiparous cows were more susceptible to biohydrogenation-induced milk fat depression in the current trial. This highlights the interactions of parity with diet composition when feeding rumen-available unsaturated fat to dairy cows.
