*4.4.2. rbGH*

As mentioned previously, GH is galactopoietic in dairy cows (Bauman et al., 1985). Since it became commercially available for use on dairy operations, the effects of rbGH on lactation performance and animal health have been extensively studied (Crooker & Otterby, 1991; Bauman, 1999; Dohoo et al., 2003). Treatment of dairy cattle with rbGH is associated with a decrease in lipogenesis and an increase in gluconeogenesis by the liver, which increases the availability of fatty acids and glucose to the mammary gland for the synthesis of milk fat and lactose (Akers, 2002). This shift in metabolism and nutrient utilization is thought to be mediated by interactions between GH, insulin, and the IGF axis (Molento et al., 2002). In addition to altered nutrient metabolism to support increased milk production, treatment with rbGH is associated with an increase in blood flow to the mammary gland (Breier et al., 1991; Prosser et al., 1996). The mechanisms underlying the milk yield response, however, remain unclear. Capuco et al. (2001; 2003) suggested that bGH increased lactation performance by increasing the population of mammary epithelial cells, possibly via the IGF signaling axis, and similar observations have been made in rodents (Allan et al., 2002). Because the milk yield response to bGH is acute and disappears upon cessation of treatment, however, it seems more likely that the mechanism works to enhance the milk synthetic activity, rather than the number, of secretory cells (Akers, 2002; Yang et al., 2005).


**Table 2.** Various management interventions that can increase milk production efficiency.

#### *4.4.3. Frequent milking*

Another management strategy that increases milk production is frequent removal of milk from the mammary gland by either suckling or increased milking frequency. Frequent milking (3 or more times daily) has been adopted on many dairy farms and has proven to be a highly successful approach to increase milk production efficiency. Reports on the effects of frequent milking on lactation persistency, however, are inconsistent. Whereas several researchers have observed an increase in persistency in response to frequent milking (Pearson et al., 1979; Poole, 1982; Amos et al., 1985; Hillerton et al., 1990; Sorensen & Knight, 2002), others have reported no effect (Allen et al., 1986; Gisi et al., 1986). The discrepancy in results may be due to differences in the definition or measure of persistency, the nature and duration of frequent milking treatment, or to the physiological state of the animals (pregnancy status, stage of lactation). Like photoperiod treatment and rbGH, the mechanisms underlying the milk yield response to increased milking frequency are unknown. It has been suggested, however, that use of the three interventions combined will elicit additive effects on milk production (Dunlap et al., 2000). This indicates that distinct mechanisms may be involved in each of the responses.

## **5. Conclusions**

274 Milk Production – An Up-to-Date Overview of Animal Nutrition, Management and Health

**Intervention Timing of implementation**  Manipulation of photoperiod Lactation; dry period Increased milking frequency Early lactation; full lactation Suckling (with or without machine milking) Early lactation; full lactation

Genetic selection Selection of cows for breeding; purchase of semen

Another management strategy that increases milk production is frequent removal of milk from the mammary gland by either suckling or increased milking frequency. Frequent milking (3 or more times daily) has been adopted on many dairy farms and has proven to be a highly successful approach to increase milk production efficiency. Reports on the effects of frequent milking on lactation persistency, however, are inconsistent. Whereas several researchers have observed an increase in persistency in response to frequent milking (Pearson et al., 1979; Poole, 1982; Amos et al., 1985; Hillerton et al., 1990; Sorensen & Knight, 2002), others have reported no effect (Allen et al., 1986; Gisi et al., 1986). The discrepancy in results may be due to differences in the definition or measure of persistency, the nature and duration of frequent milking treatment, or to the physiological state of the animals (pregnancy status, stage of lactation). Like photoperiod treatment and rbGH, the mechanisms underlying the milk yield response to increased milking frequency are unknown. It has been suggested, however, that use of the three interventions combined will elicit additive effects on milk production (Dunlap et al., 2000). This indicates that distinct

**Table 2.** Various management interventions that can increase milk production efficiency.

rBST Lactation

mechanisms may be involved in each of the responses.

*4.4.3. Frequent milking* 

As mentioned previously, GH is galactopoietic in dairy cows (Bauman et al., 1985). Since it became commercially available for use on dairy operations, the effects of rbGH on lactation performance and animal health have been extensively studied (Crooker & Otterby, 1991; Bauman, 1999; Dohoo et al., 2003). Treatment of dairy cattle with rbGH is associated with a decrease in lipogenesis and an increase in gluconeogenesis by the liver, which increases the availability of fatty acids and glucose to the mammary gland for the synthesis of milk fat and lactose (Akers, 2002). This shift in metabolism and nutrient utilization is thought to be mediated by interactions between GH, insulin, and the IGF axis (Molento et al., 2002). In addition to altered nutrient metabolism to support increased milk production, treatment with rbGH is associated with an increase in blood flow to the mammary gland (Breier et al., 1991; Prosser et al., 1996). The mechanisms underlying the milk yield response, however, remain unclear. Capuco et al. (2001; 2003) suggested that bGH increased lactation performance by increasing the population of mammary epithelial cells, possibly via the IGF signaling axis, and similar observations have been made in rodents (Allan et al., 2002). Because the milk yield response to bGH is acute and disappears upon cessation of treatment, however, it seems more likely that the mechanism works to enhance the milk synthetic activity, rather than the number, of secretory cells (Akers, 2002; Yang et al., 2005).

*4.4.2. rbGH* 

Milk production potential is dependent on the number of secretory cells in the mammary gland, as well as the metabolic activity of those cells. Both of these factors are greatly influenced by the endocrine system, by local regulatory mechanisms within the mammary gland, and by the interaction between endocrine and local regulation. Moreover, interventions that perturb the endocrine system or the local mammary environment can result in changes in mammary cell number, secretory activity, and consequent milk production potential. The mechanisms underlying the response of the mammary gland to those interventions are unknown. Research focused on determining the mechanisms involved will improve the knowledge of mammary gland biology and regulation of mammary function, and could lead to novel management strategies to further optimize milk production efficiency. Our companion chapter provides an extensive review of the literature on frequent milking or suckling as they influence milk production and mammary function in dairy animals.
