Effect of artificial or natural rearing of replacement ewe lambs on their maternal behaviors

Effect of artificial or natural rearing of replacement ewe lambs on their maternal behaviors

Summary

Adequate maternal care prepares a young animal for challenges later in life. Inadequate maternal care has been linked to increased behavioral stress response in primates and rodents, and to increased endocrine response in rodents. In sheep, maternal deprivation has been shown to increase cortisol response in young lambs, but the effects on adult ewes and their maternal behavior have not been examined.
The role of maternal care in sheep farming is important because lamb losses before weaning and increased labor around lambing time increase financial burdens and anxiety for sheep farmers. Understanding the biology of maternal behavior can help improve lambing time management and reduce stress.
Ewes with higher levels of estradiol display better maternal behavior, while cortisol levels are increased at parturition in ewes showing decreased lamb grooming. This project will use 24 artificially-reared (having received little or no maternal care) ewes and 24 naturally-reared ewes. Each of these groups will be half (12 ewes) first-time lambers and half experienced ewes. Prior to breeding, frequent blood samples will be taken to determine baseline estradiol and cortisol and then, after dog stress, to measure elevated cortisol. These ewes will be bred and blood samples again taken prior to and following lambing. Behavioral measures of maternal care will be obtained from video recorded immediately following parturition. Ewes that were artificially reared should show increased cortisol responses to dog stress and to the stress of parturition, and should show corresponding deficits in maternal grooming. Positive correlations are expected between maternal grooming and estradiol levels both before breeding and immediately prior to parturition, while a negative correlation with grooming is expected with increasing cortisol levels, confirming previous work in other breeds.
An improved understanding of the role played by received maternal care in later stress response and how this relates to maternal care at parturition will provide benefits not only for farmers dependent on lamb crop income, but for the wider scientific understanding of stress and maternal care.

Objectives/Performance Targets

• • Using a 2×2 factorial design including ewes that were artificially reared or naturally reared, and are nulliparous or multiparous:

• • To measure and compare circulating estradiol and progesterone levels prior to breeding, prior to lambing, and after lambing.

• • To measure and compare circulating cortisol levels prior to breeding, after behavioral stress, and at lambing.

• • To measure expression of maternal behavior just after lambing.

• • To look for correlations between the above-described measurements. 

It is expected that artificially reared ewes will show greater physiological responses to stress compared to naturally reared ewes. Nulliparous ewes are expected to show decreased maternal care compared to multiparous ewes, but show no differences in circulating estradiol and progesterone levels. Ewes that show heightened physiological stress responses prior to breeding are expected also to show increased stress responses at lambing, and these responses are predicted to be negatively correlated with quantity of maternal care.

Currently, ewes have been hormonally synchronized and blood plasma samples have been collected for all ewes prior to breeding. The behavioral stress (dog stress) test has been completed, with blood samples taken at ten minutes and sixty after stress to measure elevated cortisol and cortisol levels after recovery. Ewes have been exposed to intact rams for breeding and will lamb during April 2014.

It is expected that artificially reared ewes will show greater physiological responses to stress compared to naturally reared ewes. Nulliparous ewes are expected to show decreased maternal care compared to multiparous ewes, but show no differences in circulating estradiol and progesterone levels. Ewes that show heightened physiological stress responses prior to breeding are expected also to show increased stress responses at lambing, and these responses are predicted to be negatively correlated with quantity of maternal care.

Currently, ewes have been hormonally synchronized and blood plasma samples have been collected for all ewes prior to breeding. The behavioral stress (dog stress) test has been completed, with blood samples taken at ten minutes and sixty after stress to measure elevated cortisol and cortisol levels after recovery. Ewes have been exposed to intact rams for breeding and will lamb during April 2014.

Accomplishments/Milestones

In the early Fall of 2013, I prepared to begin the first blood collection for my project. Choosing and ordering the correct medical devices and accessories began as soon as grant funds were approved. This included selecting tools such as catheters and extensions, ports, suture, anticoagulant-saline flush, veterinary wrap and betadine solution. The selection of appropriate animals for my study began months before, in early Spring. I spoke with numerous professionals for informal advice as well as formal training on topics ranging from choosing a reproductive synchronization protocol to installing and maintaining indwelling jugular catheters. Creating a detailed schedule to which undergraduate assistants would be assigned was a surprisingly involved task, as was coordinating their time commitments such that I had the help I needed, when I needed it, around the clock.

The first experimental block of ewes were brought to campus during the last week in October. They departed the first week in December. In mid-to-late October, ewes were hormonally synchronized using progesterone-impregnated vaginal implants (Controlled Internal Drug Release devices, or CIDRs), and 0.6 mL of Estrumate-brand cloprostenol, a PGF-2a analogue. At the time of Estrumate dosage, 2.5-inch indwelling jugular catheters were installed in the ewes, and secured with veterinary wrap. For the 48 hours prior to anticipated onset of estrus, blood samples were drawn via these catheters. A vasectomized ram was housed with the ewes to detect the onset of estrus.

All blood samples were stored with an anticoagulant and centrifuged to obtain blood plasma, which was stored for later analysis by radioimmunoassay (RIA).

On the day following the expected onset of estrus (68 to 72 hours after CIDR removal), at the same time of day on each occasion, ewes were individually shown an unfamiliar dog. Ten minutes after dog exposure, blood samples were collected from all ewes for a measurement of peak cortisol. Ewes were returned to their home pen for the interim between blood draws and fed dry hay. Sixty minutes after dog exposure blood samples were again taken from all ewes, to show cortisol decline.

Ten days following the first CIDR removals and dosage of Estrumate, CIDRs were again reinserted in all ewes, to remain for seven days. After CIDRs were removed the second time, ewes were exposed to intact rams for breeding. A week or more after this first timed breeding period, all ewes were housed with intact rams for multiple weeks, so that as many ewes as possible were impregnated. Ewes will be scanned ultrasonically for pregnancy in mid-January 2014.

The ewes will be housed at the Cornell Sheep Farm until the week prior to their expected date of parturition. Approximately 72 hours prior to the expected onset of lambing, ewes will receive indwelling jugular catheters. Ewes will be housed indoors in individual lambing pens, and video-recorded 24 hours per day until lambing. Blood will be drawn every 4 hours prior to the onset of parturition, once immediately after parturition, and twice in the hours following. Behavioral measurements will be made from video for the 60 minutes following parturition.

Radioimmunoassay analysis of blood plasma samples will be performed in January and February of 2014, and again in April through June of 2014. Behavioral measurements will also be done in early summer, followed by data analysis.

One unexpected detail was the amount of individual variation in ewes’ responses to hormonal synchronization. One ewe appeared to be in estrus prior to removal of the CIDR and Estrumate dosage. Two ewes did not appear, behaviorally, to enter estrus. The hormonal backgrounds to these behavioral variations will be examined when RIAs are run next month.

A small change was made to the dog-stress test protocol: to ensure equal exposure of the dog to each ewe, animals were individually ushered into a 4.4 square meter enclosure, one side of which consisted of gate providing a clear view of the dog after removal of a cardboard barrier. Each ewe remained in the test pen for 60 seconds. The dog caught a tossed ball, laid down, sat up and did not interact directly with the sheep. The original protocol required that the dog to interact directly with a group of 4 sheep; modifications were made to provide for more control and equal treatment of animals. Additionally, the original protocol was overly ambitious as to the number of blood samples taken after the dog stress. The amount of time and labor required to handle the animals in this part of the experiment was intense enough that the decision was made, after review of the literature, to reduce cortisol blood samples to only at 10 and 60 minutes. These two time points are commonly presented in the stress response literature on sheep, without additional intermediate cortisol time points.

Impacts and Contributions/Outcomes

I presented an overview of my project to a group of Cooperative Extension livestock educators at the 2013 Inservice day at Cornell University. The educators were interested in the project and curious as to the results. If artificially-reared ewes prove to be poorer mothers than naturally-reared ewes, farmers can use this information to save time, money, and stress. Additionally, endocrine profiles associated with mothering behavior prior to breeding may be useful indicators of mothering ability in ewes in general.

Collaborators: