EFFECT OF FEED ENERGY LEVELS ON METABOLISM AND OVARIAN FUNCTION.

During the transition period, dry matter intake (DMI) does not generally satisfy the increasing nutrient demand of dairy cows, mainly because of a decrease in feed intake and appetite. As a result, dairy cows go into a state of negative energy balance (NEB) and thus mobilize body reserves as a physiological mechanism to adapt to the energy deficiency from late gestation to early lactation. NEB starts a few days before calving, reaching a NEB low two to four weeks post-calving, with the energy balance (EB) becoming positive again by 10 to 12 weeks after calving. During the NEB period, non-esterified fatty acids (NEFA) are released intensively from adipose tissues and recycled as energy fuels. This strong mobilization of body reserves if not properly attended to induces imbalances in fat and carbohydrate metabolism with increases in ketone bodies and tri-acyl glycerol, respectively, leading to sub- or clinical ketosis and fatty liver syndrome. A state of NEB results in the loss of body score condition (BCS) and live weight (LW). NEB could also suppress the somatotropic axis and gonadotropins on the ovaries, resulting in extended anoestrous periods. This an-ovulatory oestrus is not owing to an absence of follicular development, but to the failure of a dominant follicle to ovulate when NEB is substantial. Anoestrus is caused by low levels of metabolites (i.e. glucose) and hormones (i.e. insulin, insulin-like growth factor-I [IGF-I], luteinizing hormone (LH) and oestradiol) in the bloodstream. The decline of these and in endocrinal signals reduces the diameter and growth rate of the dominant follicle, thereby preventing or delaying it from ovulating. To address these problems, the objectives of the studies cited were two-fold: (1) to investigate the effect of nutritional treatments that differed after calving in energy levels (low versus high) and sources (starch versus fat) on blood metabolite profile and LW changes until 154 days postpartum, and to determine the extent to which these parameters could provide an indication of the post-calving nutritional status and health of Holstein dairy cows; (2) to evaluate the effects of these treatments on pre-ovulatory follicles, numbers of follicles and oestrous activity in these cows following oestrous synchronization.

During the dry period, 69 pregnant heifers and 153 dry cows were maintained under similar feeding and management conditions. After calving, cows had ad libitum access to cultivated irrigated kikuyu-ryegrass pastures, and were assigned to three concentrate groups, according to calving date, lactation number, LW and the milk yield in their previous lactation. They were supplemented with various levels and types of concentrate, of which the energy was provided by starch and fat. The Control group was offered 7 kg per cow per day of a control concentrate supplement for both first-lactation and multi-lactation groups, while concentrates in the treatment groups were fed at 11.6 and 12.6 kg per cow per day for first-lactation and multi-lactation groups, respectively. The control supplement was a maize-based concentrate, which contained low levels of starch. The concentrate components of the treatment groups consisted of high starch-low fat (HSLF) and a high starch-low fat/low starch-high fat (HSLF-LSHF) combination. The HSLF supplement was a glucogenic concentrate, which contained maize as the energy source. The HSLF-LSHF supplements consisted of a glucogenic concentrate, which was offered for the first 60 days in milk (DIM) as per the HSLF treatment, and was followed from 61 DIM with a lipogenic concentrate containing wheat bran and calcium (Ca) salts of long-chain fatty acids as the energy sources. At 80 ± 10 DIM, cows were synchronized with an Ovsynch protocol without being inseminated before the ultrasonography observation. While they were detained in a shaded neck clamp, cows were then assessed individually with an ultrasound scanner every three days for ovarian measurements and follicular activity until the subsequent oestrus.

The results showed that all cows mobilized their body fat reserves, as was evident in changes in plasma NEFA before and after calving. Post-calving NEFA and β-hydroxybutyrate (BHB) did not differ significantly between nutritional treatments in the multi-lactation cows. However, the post-calving levels of plasma NEFA and BHB were significantly higher for the control, indicating a status of advanced NEB and possible subclinical ketosis compared with the HSLF and HSLF-LSHF treatments in first-lactation cows. Post-calving plasma urea levels decreased significantly in both first-lactation and multi-lactation cows in Control, compared with the HSLF and HSLF-LSHF groups. Post-calving LW was significantly lowest and LW loss was significantly highest in Control cows compared with both HSLF and HSLF-LSHF first- and multi-lactation cows. In addition, LW loss at the low point and the number of days to reach it, significantly increased in first-lactation cows that received the control concentrate, compared to those of the HSLF and HSLF- LSHF treatments. However, this trend was not observed for multi-lactation cows. With respect to ovarian function, the results showed that ovarian and follicular measurements and the numbers of follicles in various follicle size classes were similar between nutritional treatments. However, the total ovarian follicular counts were significantly higher in cows that received the HSLF and HSLF-LSHF treatments, compared with their counterparts in Control (i.e. 7.23 ± 0.22, 7.21 ± 0.14 vs 6.53 ± 0.19, respectively).

In conclusion, the study showed that the HSLF and HSLF-LSHF treatments improved the nutritional status, as was evident in the reduced extent of NEB and the limited LW loss of these cows, compared with those in the control group. In addition, the number of ovarian follicles was also improved in comparison to the Control, possibly due to the increased energy intake. Since there was no difference between the HSLF and HSLF-LSHF groups, the results suggest that the differences with Control were due to energy level rather than energy source.