To our knowledge, this is the first study that evaluated the rbST effects on the productive performance, and health and reproductive parameters of crossbred Holstein x Gyr cows (¾ and 7/8) using different dosages (250 and 500 mg), start of treatment (40 or 63 DIM) and application interval (12 or 14 d).
In the present study, the increase in milk production in the rbST 500 and the lack of response in the rbST 250 suggest that the 250 mg dose of rbST was not able to provide the minimum daily concentration necessary to stimulate the response productive. Initial studies with daily administration of rbST observed a dose-dependent response with an increase in milk production increasing and proportional to the rbST increase until the formation of a plateau (Bauman et al., 1985; Hartnell et al., 1991).
Assuming that rbST is released uniformly and completely, the daily dose of rbST used in our study would be 17.9, 20.8, 35.7 and 41.7 mg/d for treatments 250/14, 250/12, 500/14 and 500 /12 (dose/application interval), respectively. The difference in the daily concentration of rbST released between rbST 250 (approximately 3 mg/d) allowed an increase of 2.3 kg of milk/cow/d, while the increase in rbST concentration of 20, 8 mg/d (250/12) to 35.7 mg/d (500/14) resulted in an increment of 1.3 kg. This reduction and the absence of an increase with an increase in the daily dose to 41.7 mg/d (500/12) suggests the formation of a plateau from the dose of 35.7 mg/d. The response indicates the possible existence of an optimal intermediate dose between the somatotropin concentration of 20.8 and 35.7 mg/d, and the need for further investigations to more accurately to determine the adequate daily dose for crossbred Holstein-Gyr cows (¾ and 7/8).
On the other hand, assuming the lack of uniformity in the release of somatotropin in slow-release formulations, there is a possibility of the existence of a time when the daily concentration of rbST was null or very low amount. Morais et al. (2017) reported an increase in milk production from the 2nd to the 11th d after application of rbST – vitamin E, when the rbST was performed every 14 d. Such observation suggests the absence or a low daily dose of rbST in the last 3 d of the application cycle. Thus, the lower milk production observed for the 250/14 treatment (dose/application interval) could be related to a period in which the released somatotropin concentration was null or very low, leading to interruption of rbST actions.
In our study, the increase in milk production of rbST 500 (7.3% or 1.4 kg cow/d) was lower than values reported in the scientific literature (10 to 15%, equivalent to 2–6 kg cow /d, on average) (Bauman, et al., 1999; Chilliard et al., 2002; St.-Pierre et al., 2014). Most of the studies available in the literature were conducted with Holstein cows, with greater productive capacity compared to animals from crossing with Gyr. Cows with greater genetic merit for milk production naturally have a higher concentration of endogenous somatotropin (Bauman, 1999), responsible for positively regulating the concentration of hepatic GH receptors (Gluckman e Breier., 1987). Given the results of the present study, this fact could explain the lack of productive response to the dose of rbST 250 and the smaller increase in production observed for the dose of rbST 500, since we use animals of lesser genetic merit and possibly, with lower availability of GH receptors. Our findings corroborate Fontes et al. (1997) and Phipps et al. (1997). These authors did not observe an increase in milk production with an increase in the dose from 250 to 500 mg - rbST and from 334 to 500 mg - rbST in crossbred cows (Bos taurus x Bos indicus).
In addition to having higher milk production, rbST 500 had higher FCM and ECM production. Similar results were observed by Fike et al. (2002), that reported an increase in milk production of 9% (kg cow/d) and 12% in milk production 4% fat-corrected milk, however, in Holstein cows, and under grazing conditions.
The start period of rbST treatment in Holstein cows is directly related to the successful response to hormone. Cows in early lactation through the process of negative energy balance and have a lower number of GHR-1A receptors, which contributes to a reduction in circulating concentrations of IGF-1 and lower responses in milk production (Radcliff et al., 2006). In our study, treatments were initiated after the milk production peak of crossbred Holstein-Gyr (¾ and 7/8), which occurs around 30 DIM (Pereira et al., 2016). After this moment, the GHR-1A hepatic expression returns to prepartum values, and the animals are already able to respond to the treatment (Radcliff et al., 2006). Thus, in our study, regardless of the period of start application, the animals were in similar physiological periods.
In our study, there was no change in the levels of fat, protein, lactose, and total solids between treatments. This is possibly due the fact that the animals were in a positive balance of nutrients since there were also no differences in BW and BCS between treatments. The use of rbST does not change the nutritional composition of milk in positive nutrient balance cows (Tarazon-Herrera et al., 2000 e Vicini et al., 2008). In turn, the synthesis of milk constituents increases as a function of the production volume, without changing its contents, which proves the homeorrhetic effects of rbST (Bauman, 1999).
In a meta-analysis regarding the effects of rbST in cattle, St.-Pierre et al. (2014) found no difference in BCS in treated animals, as well as in this study. Morais et al. (2017) reported an increase in BCS of less than 0.5 (scale 1 to 5) in treated animals every 14 d, and this difference was considered clinically irrelevant.
In Holstein cows, the use of rbST is responsible for increasing milk production, in addition to providing an increase in lactation persistence (Bauman, 1992; Van Amburgh et al., 1997; Morais et al., 2017). However, we were not observed an increase in lactation persistence, possibly due to the genetic composition of our animals, since crossbred Holstein x Gyr have a lower and earlier lactation peak, lower milk production, and less lactation persistence than animals of the Holstein breed (Fontes et al., 1997; Glória et al., 2010, Pereira et al., 2016).
As in the present work, most studies did not report a direct effect of rbST on SCC (Collier et al., 2001; St.-Pierre et al., 2014; Morais et al. 2017). In contrast, Bauman et al. (1999) observed a slight SCC increase in treated cows and attributed this result to the positive correlation between the SCC and milk production increased.
There was no difference in the incidence of clinical mastitis up to 250 DIM between treatments, as reported by St.-Pierre et al. (2014) in a meta-analysis. However, we were observed the interaction dose x start application, in which a higher occurrence of clinical mastitis was observed in rbST 500/40 DIM compared to rbST 250/40 (17 vs. 8 cases, respectively). The increase in clinical mastitis can be mainly attributed to two factors: an increase in milk production in response to the rbST 500 (White et al., 1994), and start applications in one period still considered at higher risk for the occurrence of intramammary infections, before to 100 DIM (Oliveira et al., 2013).
St.-Pierre et al. (2014) reported that the rbST did not change the number of open days and services per pregnancy, which reduced the pregnancy ratio by 10.5% for multiparous. In our study, we were observed negative effects for open days and number of services per pregnancy, possibly attributed to increased milk production in response to the rbST 500. The increased production is associated with a greater level of hormones metabolization, such as estradiol and progesterone, related to the suppression of estrus expression (Wiltbank et al., 2004). Plus, the reproductive management adopted by the property was based only on the estrus observation, which may justify the results.
Another hypothesis would be associated with the GH and IGF-1 serum concentration. Castigliego et al. (2009) reported a progressive increase in IGF-1 concentrations throughout lactation in treated animals (500 mg – rbST), performed every 14 d. However, as opposed to the serum GH concentrations, which reduced from the middle to the end of the application cycles, the IGF-1 concentration remained over the basal concentrations throughout the period, increasing rapidly after each new application. Bilby et al. (2006a) observed the existence of an optimal threshold concentration for somatotropin and IGF-1 to exert beneficial effects on reproduction so that exceeding these thresholds could trigger negative responses.
Holstein-Gyr cows (¾ and 7/8) managed in a semi-confinement system showed better performance and health when treated with 500 mg of rbST, initiated at 63 DIM. The interaction dose x application interval indicates a possible optimal circulating rbST dose between 20.8 and 35.7 mg per day to obtain a better response. There was no effect of the treatments on SCC and cases of clinical mastitis. The treatments with 500 mg of rbST showed an increase in open days and number of services per conception, although it was not changed the conception rate on the first service.