When it comes to replacement heifers, we focus a lot of attention on getting heifers to the appropriate weight and condition for breeding. While this certainly represents a key part of the development process, continued effort is needed to capture all of the benefits from that initial effort.
Early reproductive success has major implications for lifetime productivity. Researchers from the Meat Animal Research Center and SDSU reported heifers that conceive in the first 21 days of their first breeding period remain in the herd for 0.6 to 1.2 years longer than those that conceive in the second cycle. Not surprisingly then, this early calving group of heifers have an increased average weaning weight through six calves (Figure 1; Kill et al 2012) and more total pounds weaned compared to those calving in the second period .
In most situations, fertilization rate is estimated to be 90% or greater in beef heifers, whereas first service conception rate drops to 60-70%. While a certain amount of this early embryonic loss will likely always occur, managers want to make every effort to minimize it. Nutrition and management early after conception can influence that loss.
In one study, forage allotment was such that heifers either received 2 times their maintenance energy requirement or 80% of maintenance requirements 10 days before AI and 14 days after AI to create 4 treatments, H-H, H-L, L-L, L-H. Embryo survival rate was significantly lower for heifers that were in the H-L group (38%) compared to H-H, L-L or L-H groups (65%, 70% and 71% respectively; Duane et al., 1999).
Another study collected embryos from heifers that either continued on the pre-breeding diet (gain of 1.5 lb/d) or were fed to lose weight for 6 days until embryos were collected. Embryos that developed in heifers losing weight were behind in developmental stage, received lower quality scores, and had fewer live cells compared to those from heifers continuing to gain weight. Additional evidence that fairly short term nutritional changes can have significant impacts on embryos.
In many operations, replacement heifers are grown and developed in a drylot setting and then turned out onto spring pasture after an AI program or as the breeding season starts. Any grazing skills that they learned as a calf may have not been practiced since weaning. A recent study used pedometers to track movement of heifers that were either in a drylot or on spring pasture 42 days prior to AI. Prior to AI, heifers on pasture took twice as many steps per day as those in the drylot. When placed in a common pasture after AI, drylot heifers took nearly double the number of steps as pasture heifers the first day of turn out and took several days to reach a similar number of steps as pasture heifers.
Increased activity could relate to weight differences observed in similar situations. Weight change the first week of spring pasture was reported to be a gain of 1.9 lbs/day for heifers wintered on range compared to a loss of 3.4 lbs/day for heifers wintered in a drylot. In 4 studies comparing range vs drylot developed heifers, no statistical differences were found in pregnancy rates. However, drylot developed heifers that receive supplementation the first month of grazing following AI, had higher pregnancy rates than non-supplemented heifers.
When reviewing these data, the take home message may be particularly profound in our current drought conditions. Feed supplies of any type for the wintering period may be limited and push producers to make hard choices depending on when spring grazing is available. If the quantity of forage is short and/or heifers have to learn to graze, a temporary decrease in performance is likely to occur. Small, relatively short term changes in nutrition can negatively impact embryo survival in heifers. Heifers would be most sensitive to these changes from day 5, when the embryo first enters the uterus, to day 42, when the embryo is fully attached to the uterus. When weighing the costs of alternatives, do consider the impact of early pregnancy the first season on lifetime productivity.
Source: Sandy Johnson, livestock specialist