The Heifer Project has been tracing the effect of body weight and body condition on heifer fertility for over ten years. Approximately 336 heifers are monitored each year with the animals being weighed at the beginning and the end of the grazing season. These measurements are then compared to the fall preg test results and, over the past three years, treatment records (foot rot etc) to determine their impacts on conception rate.
For the past few years the heifers have been weighed two additional times, when they are switched from tame pasture to native brush pastures around the end of July and then when they switch from these native pastures back to the tame pastures around mid-September. The lease currently operates with 12 paddocks (figure 1) with a typical grazing rotation illustrated in Table 1.
Figure 1. Map of the Northern Range Enhancement Project (NREP) pasture system.
Table 1. Typical grazing rotation for the Olympic Lake Grazing Lease
In 2009 in drought conditions, there was higher weight gain on the native bush pastures. Just as in 2002, the heifers spent more time in the bush because of the lack of moisture and limited re-growth of the tame pastures. In 2010, weight gain was higher on the tame pastures, with very good gains occurring on the tame re-growth late in the season. Similarly, 2011 showed good gains on the native bush pastures although fall regrowth was poorer as a result of weather conditions and lack of rain after the end of July. There were also water issues late in the 2011 grazing season and heifers had to drink right out of the dug-out for the first time which could also have impacted gains. In 2013, heifers showed a good rate of gain for season. However, as a result of only one additional weigh between take-in and take-out a comparison of gain on tame pasture vs. native bush pasture could not be shown. Similar to 2011, lack of rain after July contributed to water issues, which may have impacted heifer weight gains towards the end of the grazing rotation.
Table 2. Historical data from Olympic Lake Grazing lease, 2003-2014.
Since 2011, records have been kept on which animals were treated during the breeding season and this information was compared to pregnancy rate at the end of the pasture season to see if treating or the stress of being lame or sick would negatively impact conception.
The 2011 results showed a negative correlation between the open rate of the whole herd of 350 animals (% open) and the number of animals treated (% treated) indicating that the higher the treatment rate the lower the open rate. However, when the average open rates between the whole herd, the untreated herd and the treated herd were compared treatment did not appear too negatively impact fertility as the treated herd had the lowest open rate. This trial was repeated in 2012 and, in contrast to 2011, the results indicated that with increased treatment rate there was an increase in open rate. Due to the conflicting data, correlations between treatment rate and conception were once again evaluated in 2013 (Table 3) with results confirming those found in 2012. However, treatment rate was low in 2013 with only 3 animals being treated during the breeding season. Consequently, sample size is low and data may not be representative as 20 or more heifers were treated in 2011 and 2012.
Data was once again collected in 2014 with a larger sample size of 14 heifers being treated. The results agreed with those found in 2012 where an increase in treatment rate showed an increase in open rate.
Table 3. Treatment and pregnancy data, 2014.
Correlation between whole herd % open, and whole herd % treated = 0.597534992
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