Genetics plays a big part.
Getting lactating cows in calf has never been easy. It has become even harder over the past 25 years.
* US: inter-calving interval increased by one month and services per conception increased 33 per cent
* Ireland: services per conception increased by 14 per cent
* UK: calving rate to first insemination declined from 56to 40per cent
* New Zealand: six-week re-calving rate declined from 70to 50 per cent.
These studies indicate a reduction in conception rate and an increase in embryo mortality over the past 25 years, while longer post-calving anoestrous intervals and reduced expression of heat have also contributed to the decline. During the same period, milk production/cow has increased and cows now tend to lose more BCS in early lactation. Because of this, many people have associated failure to get cows in calf with negative energy balance in early lactation. They assume that feeding cows more pasture or feeding particular supplements in early lactation will improve reproduction.
Reproductive failure - influence of genetics
Comparisons between New Zealand cows and those of North American ancestry offered the same diet have proven that genetics has a strong impact on reproductive failure. New Zealand cows have higher conception rates and supplementary feeds offered to cows well fed on pasture do not correct the poor reproductive performance of the North American cow. Experiments at DairyNZ and internationally have discovered that there are important differences between these strains in the:
* length of their reproductive cycle
* concentration of important hormones circulating in blood
* expression of key genes in the uterus that enable the embryo to grow and survive.
It is unlikely that these genetic effects can be overcome by nutrition.
Reproductive failure - influence of nutrition
Many nutritional factors have been suggested as contributing to decline in fertility in New Zealand.
Body condition score: Body condition score at calving is arguably the most important nutritional factor associated with getting cows pregnant. Cows that are fatter at calving, cycle earlier and tend to be fatter at mating. However, cows that are too fat lose excessive condition after calving and are less likely to conceive. It is recommended that mature cows calve at BCS 5; this ensures they cycle early, lose on average no more than 1 BCS unit between calving and mating, and are greater than BCS 4 at mating.
Younger cows (heifers and second calvers) tend to be healthier and less prone to disease (mastitis and endometritis), and they cycle earlier if they calve a little fatter than mature cows (BCS 5.5). It is important to pay attention to nutrition during late lactation and the dry period to ensure cows reach recommended BCS targets. Failing to get cows to target condition at calving cannot be corrected by nutrition in early lactation.
Transition period: Nutrition of the springing cow before calving influences liver health after calving. This may affect the incidence of metabolic diseases and uterine infections, which can affect reproductive performance. Management of the cow during the transition period was recently profiled in the June issue of Technical Series available at dairynz.co.nz/techseriesjune15.
Intake: Many believe that cows fed only pasture cannot eat enough to meet demand and that supplements will therefore improve energy balance. It is true that cows cannot eat sufficient DM in early lactation to meet energy requirements for milk production; they will be in negative energy balance and will therefore lose BCS. This is primarily controlled by genetics, and feed amount or feed type have little effect on BCS loss in the first four to five weeks of lactation.
Although a negative energy balance during mating will reduce the likelihood of a cow getting in calf, the effect on fertility is not as great as many think. DairyNZ data indicates a reduction of 4 per cent in six-week in-calf rate if cows lose 2 BCS units during early lactation compared with cows that lose 1 unit.
Furthermore, in a large study in which cows had a 40 to 50 per cent restriction imposed for the first two weeks of mating, these cows had a 6-7 per cent lower pregnancy rate to first service and six-week in-calf rate. Although such a decline in fertility is important, this was a very severe restriction. Results indicate that feeding level in early lactation is not the main reason for poor fertility and that supplementation, per se, will not improve in-calf rates. Supplementation can influence BCS from week six of lactation onwards, but the effect is small.
Technical Series, DairyNZ