Effect of rate of gain during early gestation on colostrum and milk composition in beef heifers

We hypothesized that rate of gain during the first 84 days of gestation would affect composition of colostrum and milk, and increase milk production in moderate-gain heifers. At breeding, 45 Angus-based heifers received either a basal total mixed ration allowing 0.63 pounds per day (lb/d) gain (low gain [LG], n = 23) or basal diet plus starch-based supplement allowing 1.75 lb/d gain (moderate gain [MG], n = 22) for 84 days. Heifers then were managed on a common diet until parturition. Colostrum samples (50 milliliters [mL]) were collected before first suckling. Milk samples (50 mL) were collected six hours after calf removal on days 62 ± 10 and 103 ± 10 postpartum. Samples were collected by stripping each teat 15 to 20 times after discarding the first five strips. At day 103, sampling techniques were compared by collecting a second sample after 1 mL oxytocin administration and 90 seconds lag time. The colostrum somatic cell count (SCC) was greater (P = 0.05) in LG (6,949 ± 739 cells × 103/mL) than MG (4,776 ± 796 cells × 103/mL). In milk, protein and other solids were greater (P ≤ 0.03) in MG (3.02 ± 0.03 and 6.20 ± 0.02 %, respectively) than LG (2.87 ± 0.03 and 6.14 ± 0.02 %, respectively). On day 103, oxytocin administration and extended lag time after teat stimulation (0.96 ± 0.05 %) increased fat concentration in milk (P < 0.01), compared with immediate milk sample collection (0.34 ± 0.05 %). We conclude that nutrition during early gestation had a sustained impact on milk composition, and techniques of oxytocin administration result in greater milk fat content.

In cattle, the development of the mammary gland begins during embryonic development, with the majority of its growth occurring during the last trimester of gestation. By parturition, all components of the gland are established in the fetus, including vascular, lymphatic, connective and adipose tissues (Rowson et al., 2012). In the heifer dam, the majority of apparent mammary growth occurs during the last trimester of gestation and is completed at parturition (Rowson et al., 2012).

Therefore, optimal development and growth of the mammary gland during gestation is essential to ensure maximized milk production in future lactations. Additionally, the mammary gland is a key tissue ensuring the transfer of nutrients and immunoglobulins to the neonatal calf (Neville et al., 2010). Because of the importance of a dam’s milk production on her calf’s weaning weight, optimizing milking potential is crucial.

Milk is produced in the secretory tissue of the alveoli; however, milk’s nutritional constituents and consequently, composition, vary depending on place of storage in the udder. In contrast to casein micelles (protein), which are small enough to passively transfer from the alveoli into the cistern, milk fat globules are larger and require active expulsion from the alveoli. Therefore, fat content is greater in the alveoli than in the cistern, whereas protein content is similar across the two storage sites.

Milk letdown is initiated by oxytocin, which is released from the pituitary gland in response to tactile stimulation of the udder, and causes the myoepithelial cells around the alveoli to contract and eject the milk stored there into the duct system and cistern (Bruckmaier and Blum, 1998). However, an approximate one- to two-minute lag period occurs between the release of oxytocin and milk expulsion (Bruckmaier and Blum, 1998).

Based on the lack of knowledge that we encountered in the literature regarding maternal nutrition during early gestation and its effects on lactation, we aimed to evaluate the impact of low and moderate gain during the first 84 days of gestation on composition of colostrum and milk, and milk production.

All animal procedures were approved by the Institutional Animal Care and Use Committee at North Dakota State University.

Forty-five Angus-based heifers (initial body weight [BW] = 818.2 ± 8.7 pounds) were estrus synchronized using a Select Synch plus CIDR protocol and bred via artificial insemination to female sexed semen from a single sire. At breeding, heifers were blocked by antral follicle count, ranked by BW and assigned to one of two treatments: 1) a basal total mixed ration (TMR; low gain [LG] 0.63 lb/d; n = 23) or 2) the basal TMR diet with the addition of a starch-based energy/ protein supplement mixed into the diet (moderate gain [MG] 1.75 lb/d; n = 25, Table 1).

1 - Low gain: Heifers fed a basal total mixed ration (TMR) contained a commercially available mineral supplement (Purina® Wind & Rain® Storm® All-Season 7.5 Complete Mineral, Land O’Lakes Inc., Arden Hills, Minn.) fed at a rate of 4 ounces per head per day, targeting gain of 0.63 lb/d.
2 - Moderate gain: Heifers fed basal TMR plus an energy/protein supplement formulated with a blend of ground corn, DDGS, wheat midds, fish oil and urea, targeting gain of 1.75 lb/d.

Heifers were fed individually using the Insentec Feeding System (Hokofarm B.V., Marknesse, The Netherlands). Heifers were weighed on two consecutive days at the beginning and end of the feeding trial, and every 14 days throughout the 84-day period prior to morning feeding, then on days 164, 234 and 262 and at the time of calving, pasture turnout and weaning.

At calving, a 50-mL colostrum sample was collected from each heifer, before calves suckled for the first time. For sample collection, we stripped each teat 15 to 20 times after discarding the first five strips. At day 62 ± 10 postpartum, we estimated milk production using a 12-hour weigh-suckle-weigh procedure. Briefly, dams and calves were assigned to two groups of 23 and 22 pairs each. At midnight, we separated calves from their dams. At 6 a.m. the next morning, calves were allowed to nurse their dams until satiety (about 30 minutes) to establish similar milking status across the dams.

Then, pairs were separated for two six-hour time periods. After each six-hour window, calves were weighed before and immediately after suckling until satiety (about 30 minutes). The difference between the pre- and post-suckling calf weights was recorded as the estimated milk production of the dam for each of the six-hour time periods.

To estimate 24-hour milk production, milk production for the two six-hour separation periods was added together and multiplied by 2 (Shee et al., 2016). Before allowing the calves to suckle their dams at 6 a.m., we collected a 50-mL milk sample into DHIA vials by stripping each teat 15 to 20 times after discarding the first five strips. Samples were mixed thoroughly and stored at 4 C until further analysis.

At day 103 ± 10 postpartum, the same protocol was used as at day 62 postpartum. Immediately following the collection of the milk sample, we administered oxytocin (1 mL i.m.) to each dam and waited for 90 seconds before collection of another 50-mL milk sample to compare sampling protocols. All samples were shipped to a DHIA milk laboratory (Stearns County DHIA Lab, Sauk Centre, Minn.) within 10 days (colostrum) and five days (milk) after sample collection for analysis of composition of colostrum and milk (fat, protein, somatic cell count [SCC], milk urea nitrogen [MUN] and other solids). Heifer BW was analyzed as repeated measures in time using the MIXED procedure of SAS (SAS Inst. Inc., Cary, N.C.) for effects of treatment, day and a treatment × day interaction. Colostrum and milk data were analyzed using the GLM procedure of SAS with the effects of treatment, day/oxytocin and their interaction. Heifer was considered the experimental unit in all analyses and significance was set at P ≤ 0.05.

Heifer body weight was affected by a treatment × day interaction (P < 0.01), being similar at initiation of treatment, diverging by day 14 (P = 0.01) and was 122.1 lbs. greater for MG heifers at day 84 (P < 0.01; Figure 1).

Figure 1. Impact of nutritional treatment on body weight of heifers managed at
two rates of gain (low gain [LG], 0.63 lb/d; moderate gain [MG], 1.75 lb/d) for 84
days, followed by common management for the duration of gestation and lactation.
*Within day treatments differ (P < 0.01), † with day treatment differ (P = 0.04).

Although heifers were managed as a single group beginning at day 85, the weight divergence continued throughout calving until weaning, at which times heifers in the MG treatment remained 90.4 pounds (P < 0.01) and 56.1 pounds (P = 0.04) heavier than LG heifers, respectively.

In colostrum (Table 2), we observed an effect of maternal treatment on SCC (P = 0.05), which was lower in MG heifers than in LG heifers; however, the percent of fat (P = 0.11), protein (P = 0.40), other solids (P = 0.17) and MUN (P = 0.29) were not influenced by rate of gain during the first 84 days of gestation. Somatic cells in colostrum and milk include epithelial cells and leukocytes (macrophages, neutrophils and lymphocytes), with the majority of somatic cells in milk being leukocytes (Kelly et al., 2000).

1 - Treatment: Low-gain heifer (LG) fed a basal TMR contained a commercially available mineral supplement targeting gain of 0.63 lb/d; moderate-gain heifers (MG) fed basal TMR plus an energy/protein supplement targeting gain of 1.75 lb/d
2 - SEM = Standard error of the mean (LG, n = 23; MG, n = 22).
3 - Values for other solids include lactose and ash.

Consequently, SCC is an indicator of colostrum and milk quality, and a measure of inflammation and infection in the udder. Somatic cell score is greater in colostrum than in milk, which may be caused by cells passing through leaky tight junctions present in the mammary epithelium, which close when milk production increases (McGrath et al., 2016).

Maternal dietary treatment did not affect milk production on day 62 postpartum (P = 0.67; LG: 10.6 ± 0.91 pounds/day; MG: 11.2 ± 0.92 pounds/day), but influenced milk composition on days 62 and 103 postpartum (Table 3). Moderate-gain heifers had a greater percentage of milk protein (P < 0.01) and other solids (P = 0.03) than LG heifers.

1 - Low gain: Heifers fed a basal TMR contained a commercially available mineral supplement targeting gain of 0.63 lb/d.
2 - Moderate gain: Heifers fed basal TMR plus an energy/protein supplement targeting gain of 1.75 lb/d.
3 - SEM = Standard error of the mean (LG, n = 23; MG, n = 22).
4 - Values for other solids include lactose and ash.
5 - Milk sample collected at day 62 ± 10 postpartum.
6 - Milk sample collected at day 103 ± 10 postpartum.

Further, the percent of fat and other solids in milk decreased from day 62 to day 103 postpartum (P < 0.01), whereas the percent of protein in milk and MUN increased for the same time periods (P < 0.01). This could be related to the nutritional management of the heifers, as milk composition can be influenced by multiple factors, with milk fat being the component that can vary the most as a result of environmental and physiological factors, especially nutrition (Bauman and Griinari, 2001). Here, heifers received a basal TMR in a dry-lot setting at day 62, whereas they were grazing native range at day 103 postpartum.

At day 103 postpartum, using a sampling technique that included oxytocin administration and an extended lag time of 90 seconds after teat stimulation, we saw an increased percent of milk fat (P < 0.01), compared with collecting an immediate sample without oxytocin injection (Table 4). However, oxytocin administration and the extended lag time did not affect the percent of milk protein (P = 0.98).

1 - Low gain: Heifers fed a basal TMR contained a commercially available mineral supplement targeting gain of 0.63 lb/d.
2 - Moderate gain: Heifers fed basal TMR plus an energy/protein supplement targeting gain of 1.75 lb/d.
3 - SEM = Standard error of the mean (LG, n = 23; MG, n = 22).
4 - Milk sample collected before injection of 1 mL of oxytocin and a 90-second lag time.
5 - Milk sample collected after administration of 1 mL of oxytocin and a 90-second lag time.

Both observations make sense in regard to the anatomy of the mammary gland and the role that oxytocin plays in the milk ejection process. Regardless of the sampling technique used, milk fat concentrations were extremely low and do not appear representative of the milk fat that calves have access to when compared with results by Kennedy et al. (2019), who reported fat concentrations greater 4% in beef cows (4.11 ± 0.33% for control and 4.21 ± 0.33% for supplement). Therefore, future sampling techniques should focus on milking at minimum an entire quarter to obtain a better representation of nutrients in milk.

The authors express their gratitude for the many personnel involved in this project including staff at the Central Grasslands Research Extension Center and the Beef Cattle Research Complex, and undergraduate students for assisting with animal handling and data collection.

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