EFFECTS OF A PROTECTED FAT SUPPLEMENT ON THE GNRH INDUCED LH RELEASE IN EARLY POSTPARTUM BEEF COWS AND OVARIECTOMIZED BEEF COWS

Abstract

Two trials were conducted to study the effect of feeding a protected-fat supplement on reproduction. In the first trial eight ovariectomized brangus cows were used to study the effect of feeding a protected-fat supplement on LH release after GnRH injection. The cows were equally divided at random into two groups (control and treatment), placed in individual pens, and fed a ration supplying 4.0 kg of TDN per head daily. Cows in the treatment group received a .68 kg of protected fat daily for the entire experimental period (30 days). On the last day of the experiment an indwelling jugular catheter was inserted into the cows and blood samples were taken every ten minutes for a period of 5 hours. After the sixth sample was taken, 200 (mu)g of GnRH were injected intramuscularly to induce LH release. The samples taken before the GnRH was injected were used to establish basal LH levels. LH levels were analyzed by using the double antibody radioimmunoassay technique. Average weight of the cows at the start of the trial for the control and treatment group were 491 and 457 kg respectively. Cows in the control group lost weight (-8 kg) during the experimental period, whereas those in the treatment group gained 33 kg. The highest LH peak value was 12 ng/ml for the controls and 43 ng/ml for the treated cows. Mean LH values were 4.9 ng/ml for the control group and 43 ng/ml for the treatment group. In the second experiment twenty multiparous four-year-old pregnant brangus cows are used to determine the effect of a protected fat diet on postpartum pituitary response to GnRH. As cows calved they were randomly assigned to either the control or treatment group. The treatment consisted of adding .68 kg of protected fat to the ration. The diet for both groups was designed to supply approximately 4.4 kg of TDN daily per animal. This was about 80% of the NRC requirement. On day 7 postpartum an indwelling catheter was inserted into the jugular vein. Blood samples were taken for 5 hours every ten minutes. After the sixth sample was taken 200 (mu)g of GnRH were injected intramuscularly to induce LH release. The pre-GnRH injection period was used to establish basal LH levels. Luteinizing hormone levels were analyzed using the radioimmunoassay double antibody technique. The latter part of this experiment was designed to assess the effect of a protected fat diet on postpartum estrus activity. Estrus was also checked by daily visual observation. Average weight of the cows at the start of the experiment was 514 and 474 kg for the control and the treatment group respectively. By the end of the trial (75th day) cows in the control group lost 35 kg, and those in the treatment group lost 22 kg. Average daily gain of calves whose mothers were on treatment or control did not differ. Mean LH levels were 18 and 13 ng/ml for the treatment and control cows respectively. This difference was not statistically significant due to a large mean standard error. Fifty percent of the cows receiving the protected fat supplement had shown standing estrus by day 45 postpartum vs 20% for the control group. By day 15 pospartum 60% of the cows in the treatment group had shown signs of postpartum estrus activity vs only 10% for the control group. By day 45 postpartum all the cows receiving the protected fat supplement had shown signs of estrus activity vs only 50% for the control.