Dairy heifers: does it matter if they drop below target weight over winter?
Updated: Jun 2, 2020
Do you have a growth plan for your heifers this winter? Recent research out of Massey University suggests that a small drop below target weight over winter had limited disadvantages to replacement heifers, so long as they caught up to target by mating.
Read below for a summary on the research from lead author, Dr Rhiannon Handcock.
No differences in pregnancy rates, date of calving or first lactation milk production
These results suggest there were limited disadvantages to growing heifers slower over their first winter, provided they caught up to target weight by first mating.
What are the industry recommended target weights for dairy heifers?
The target weights of dairy heifers in NZ are:
• 30% of mature weight at 6 months of age
• 60% of mature weight at 15 months of age (first mating)
• 90% of mature weight at 22 months of age (before first calving)
110 replacement heifers were grown to target weight at 6, 15 and 22 months of age. From 6 months of age, heifers were split into 2 groups. This first group was named “TARGET” and the second group was named “SEASONAL”.
The Target group of heifers were grown in a straight line from 6 to 15 months of age (~0.6 kg/d) as recommended by industry. The Seasonal group were grown slow over winter and fast over spring, as is commonly seen in NZ heifers.
Both groups met target weight at 6 and 15 months of age, however, the pattern of growth was different. As shown in Figure 1 below.
What age did dairy heifers reach puberty?
The average age of puberty was 342 days for the Target group and 380 days for the Seasonal group, this equates to 38 days earlier for heifers in the Target group.
All heifers were monitored for age of puberty attainment with KAMARs and ultrasound scanning by Massey vets.
The percentage of heifers that had reached puberty by 15 months of age is displayed below in Figure 2. There was a clear difference in the age of heifers in each group when 25%, 50% and 75% had reached puberty.
This meant that more heifers in the Target group had multiple oestrous cycles before the planned start of mating (PSM) compared with the Seasonal group.
It is important to note that in animal studies when proportions/percentages are involved, (e.g. proportion pregnant, proportion cycling etc.) there needs to be large numbers of animals in each group in order to test if the two groups are different.
For example, in an imaginary study, FARM A had a 70% pregnancy rate and FARM B had a 90% pregnancy rate. A pretty substantial difference right?
However, both farms started off with only 10 cows each. So, 7/10 for Farm A vs 9/10 for Farm B. It is difficult to say that Farm B is any better that Farm A, as Farm B had only 2 more pregnant cows than Farm A.
However, if each farm had 1000 cows, this same percentage difference (70% vs 90%) would be a difference of 200 pregnant cows, much more meaningful right?
In this study on heifers, there were enough heifers in each group to compare continuous data (milk production, age at puberty, date of calving). But, unfortunately there were not enough heifers in each group to confidently say that the two groups were different (or the same) for pregnancy rates.
However, there is limited data available to the industry on the effects of falling below target weight over winter on heifer reproductive performance. Therefore, the authors took the opportunity to present the data to give an idea of the performance that can be expected. So, take it with a grain of salt!
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Did growth pattern affect pregnancy rates?
A study on beef heifers in the USA reported that 78% of heifers that were bred to their 3rd heat were pregnant, whereas, only 57% of heifers bred to their 1st heat were pregnant. Therefore, it was expected that the Target group would have higher pregnancy rates than the Seasonal group.
Surprisingly, there were no differences in the percentage pregnant in the first 3 weeks, 6 weeks or by the end of the 7-week mating period! The median calving date was the 1st August (5 days after PSC) for both groups.
For the heifers' second mating period (during first lactation) there were also no significant differences between the two groups in percentage pregnant in the first 3 or 6 weeks.
Did faster growing heifers produce more milk?
These heifers were born in 2015, so were able to be monitored until the end of their first lactation. Heifers lactated for approximately 280 days and produced over 330 kg MS.
There were no differences between the two groups in milk yield or milk composition (fat and protein percentages). See below table.
Combined, these results suggest that provided heifers reached their pre-mating target weight, falling below target over winter did not negatively impact reproductive performance or first lactation milk production of heifers.
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Handcock RC, Jenkinson CMC, Laven R, McNaughton LR, Lopez-Villalobos N, Back PJ, Hickson RE 2019. Linear versus seasonal growth of dairy heifers decreased age at puberty but did not affect first lactation milk production. New Zealand Journal of Agricultural Research