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Reproductive performance in the modern high-yielding Holstein-Friesian dairy cow has decreased at such a rate, and to such an extent, over the last 40 years that it is predicted that this high-performance production system will become unsustainable by 2020 if the current rate of decrease continues. It is considered unlikely that this decrease in reproductive performance has a direct genetic origin, as conception rates in non-lactating Holstein-Friesian heifers have remained high (at 70–80%) during a period when milk production has increased by more than 200%. In general, this large increase in milk (energy) output has not been matched by a proportionate increase in energy intake, resulting in a negative energy balance (NEB), which forces the mobilization of body reserves. Negative energy balance has been determined as an underlying causal factor of poor reproductive performance in high-yielding dairy cows, as it has been associated with impaired ovarian function, an extended interval to first service and decreased conception rates.
Several dietary components have the potential to influence the extent of NEB. For example, dietary crude protein has been shown to affect milk output and thereby influence NEB. The inclusion of protected fat in dairy cow rations increases the energy density of the diet and also has the potential to decrease the extent of the NEB. Supplementation with fat can increase cholesterol concentration, which serves as a precursor for progesterone synthesis, whereas blastocyst growth rate can also be improved by the inclusion of fat in the diet. Diets high in starch content increase the supply of glucose precursors and can increase circulating insulin concentrations which, in turn, are associated with enhanced cyclicity after calving. However, despite the key importance of nutrition, poor reproductive performance remains, and therefore Dr H.S. Gilmore and co-workers designed a study to further investigate the influence of nutritional strategies. The study was reported in the Journal of Dairy Science, Volume 94 of 2011, pages 3510 to 3526, with the title:An evaluation of the effect of altering nutrition and nutritional strategies in early lactation on reproductive performance and oestrus behaviour of high-yielding Holstein-Friesian dairy cows.
The aim of the current study was to evaluate the effect of four distinctive nutritional regimens on the reproductive performance of high-yielding Holstein-Friesian cows. The hypotheses attached to the use of the nutritional regimens applied were: (1) adjusting the protein content of individual cow diets upwards or downwards can be used to more closely align cows to an optimal and more uniform energy balance trajectory; (2) offering a high-starch diet post-calving decreases the interval from calving to the onset of ovarian activity; (3) switching cows to a low-starch/high-fat diet at 50 days post-calving improves reproductive parameters; and (4) offering a decreased protein ration supplemented with protected methionine (first-limiting amino acid) decreases plasma urea concentrations in dairy cows while sustaining milk production and improving reproductive parameters.
These nutritional regimens aimed at improving fertility were applied to 96 Holstein-Friesian cows. Upon calving, the cows were allocated in a balanced manner to one of the four dietary treatments. First-calf heifers were balanced according to live weight, body condition score and calving date. Older cows were balanced according to parity, previous lactation milk yield, live weight, body condition score and calving date. Treatment 1 was based on an industry best practice diet (control), containing 17% crude protein. Treatment 2 was an individual cow feeding strategy, whereby the energy balance of individual animals was managed so as to achieve a predetermined target daily energy balance profile of 10 megajoules (MJ) per day. Treatment 3 was a high-starch/high-fat combination treatment, whereby an insulin-promoting (high-starch) diet was offered in early lactation to encourage cyclicity and followed by a fat-supplemented (low-starch, high-fat) diet to promote embryo development. Treatment 4 was a low-protein diet, containing 14% crude protein, supplemented with protected methionine at an inclusion level of 40 g per animal per day.
The nutritional strategies had no significant effects on cow fertility measures, which included the onset of luteal activity, conception rate, in-calf rate, and the incidence of atypical cycles. The individual cow feeding strategy improved energy balance in early lactation but had no benefit on conception rate to first insemination. However, conception rate to second insemination, 100-day pregnancy rate (from the commencement of breeding), and overall pregnancy rate tended to be higher in this group. The high-starch/high-fat treatment tended to decrease the proportion of delayed ovulations and increase the proportion of animals cycling by day 50 post-calving. Animals that failed to conceive to first insemination had a significantly longer luteal phase in the first cycle postpartum and a longer inter-ovulatory interval in the second cycle postpartum. With regards to oestrous behaviour, results indicate that as the size of the sexually active group increased, the intensity of oestrus and the expression of mounting or attempting to mount another cow also increased. Furthermore, cows that became pregnant displayed more intense oestrous behaviour than cows that failed to become pregnant.
None of the nutritional regimens or interventions applied in the current study had a statistically significant effect on the reproductive performance of the high-yielding Holstein-Friesian dairy cows. In conclusion, trends were observed, but cow numbers appeared to have been limiting. With larger numbers, significance may have been achieved to verify the observed trends: improved ovarian activity in animals offered the high-/low-starch diet and improved energy balance and subsequent fertility in cows managed on an individual basis. Managing the nutrition of cows so as to adjust individual milk yields resulted in a more energetically uniform group of cows at the commencement of breeding.
The findings from this study re-emphasize the complex and multi-factorial nature of the reproductive cycle in modern high-yielding Holstein-Friesian cows. It certainly cannot only be rectified by nutritional strategy, even precision feeding.