Growth targets and rearing strategies for replacement heifers in pasture based systems: a review

This paper is interesting because it looks at how the pre-weaning growth is the most important part of the heifer rearing programme and if the heifer isn't reared properly from birth with adequate access to colostrum and feed. Large average daily gains in the pre puberty period can adversely affect the growth of mammary secretory tissue.

Low live weight heifers at 15 months are low live weight compared to the rest of the herd until at least their third lactation.

The next question this draws up is where should the mature live weight be drawn from? Should it be the genetic potential mature weight or the average herd live weight? The herd's mature weight could, therefore, be a function of historical heifer growth trajectory and projecting heifer growth on data from cows that were poorly grown as heifers would perpetuate the cycle of poor productivity.

40% calves suffer from failed passive transfer of immunoglobulins caused by a failure to feed enough colostrum quickly enough to the new born calf. Calves with FTP will have increased rates of mortality and morbidity. It also reduces DM intake and milk production later on.

It is often reported anecdotally that graziers receiving calves at weaning that the 'tail end calves' never do very well and will always be behind the rest of the mob. This paper recognises accelerated ADG in the first eight weeks is associated with enhanced mammary development and identifies further work is required to understand the relationship between early life average daily gain and gastro-intestinal development.

 It is interesting the way that different speeds of average daily gain has a different effect on mammary development at different stages of the heifers path to maturity. The links showing in the papers reviewed that an accelerated ADG pre wean has a positive impact on mammary development and subsequent lactation after the first calving. Then some studies will show a negative correlation between accelerated growth and mammary development from post weaning through to puberty.

 It is hard to achieve a linear ADG in the grazed pasture growth system. This is due to the high NDF levels in summer restricting the VFI and reducing the ME of the pasture available to the heifer and low growth rates in the winter reducing the level of feed available. The question is can the pastoral system be manipulated to match the needs of the heifer and use the natural pastoral growth cycle to feed the heifer to her own benefit and use compensatory growth to accelerate the ADG when required?

 Though stair step methods of gain using compensatory growth have been shown to have poor utilisation of energy for live weight gain and was used for lay down of fatty tissue rather than lean tissue.

J. R. Roche, N. A. Dennis, K. A. Macdonald, C. V. C. Phyn, P. R. Amer, R. R. White and J. K. Drackley (2015) Growth targets and rearing strategies for replacement heifers in pasture-based systems: a review, Animal Production Science, 55(7), 902-915

Conclusions and further research

Mature Lwt is hard to define, because it is dependent on both the genotype of the heifer and the environment in which she is managed. Furthermore, there does not appear to be a significant advantage to defining a mature Lwt for individual heifers compared with an average mature Lwt for the herd. Although accepted as dogma that heifers should calve between 80% and 90% of mature Lwt, the advantage of achieving this target is not clear. The effects of pre-pubertal and post-pubertal ADG on milk production and reproduction are inconsistent, as long as the heifer has had sufficient ADG to reach puberty before the desired breeding date. That said, there is morphological and physiological evidence that accelerated pre-pubertal ADG reduces mammary parenchymal development; what is not clear, however, is whether, or not, there is compensatory development of the mammary gland during the gestational allometric development phase. What is also not clear is whether accelerated growth trajectories result in changes to the composition of nonmammary tissue and whether this could potentially have negative effects in the calved heifer. Further research is required in these areas. Consideration should also be given to the trajectory of gain, whether periods of restriction and re-alimentation increase the amount and activity of mammary secretory tissue and whether the advantage can be exploited in pasture-based systems. Further work is required to understand why such a high proportion of calves fail to receive sufficient high-quality colostrum to ensure adequacy of immunoglobulin transfer. The importance of colostrum in the lifetime productivity of the replacement animal has been known for decades. Therefore, the failure of this vital practice on farm is worthy of social study and increased extension effort. A growing area of interest is the pre-weaning nutrition of the calf and, in particular, the first 8 weeks of life. Research has indicated a significant return on limited additional investment during this time. However, the majority of that work has been undertaken in high-production, housed dairy systems. Further research is required in grazing systems and, in particular, research efforts should focus on determining whether the window of opportunity is even shorter than has been indicated.