NUTRIENT PARTITIONING AND MILK YIELD: CONSTRAINTS AND OPPORTUNITIES IN THE 21 ST CENTURY 1

Department of Animal Sciences, University of Arizona INTRODUCTION Global prosperity continues to improve and more affluent nations traditionally demand and consume more foods of animal origin (Roche and Edmeades, 2004). Thus, worldwide consumption of dairy products is increasing, and meeting this demand requires improvements in productive efficiency. Identifying the biological processes altered to achieve these advancements are of special interest. Bauman et al. (1985), Gorden et al. (1995), and more recently Reynolds (2004) reviewed the biological basis for these improvements and found that digestion and nutrient absorption, maintenance requirements, and the partial efficiency of nutrient use for productive functions (i.e. milk synthesis) were only minor sources of the variation among animals. In contrast, animal differences in nutrient partitioning represented the major source of improvement. For example, feed intake is largely dependent on signals relating to tissue nutrient utilization; high producing cows consume more nutrients and direct these for milk synthesis rather than excessive fattening. Maintenance requirements are relatively constant regardless of milk production level. Thus, high producing cows have a greater nutrient intake in order to support additional milk production, but a larger portion of total nutrient intake is used to synthesize milk. This is often referred to as “dilution of maintenance” and represents the major basis for the remarkable gains in productive efficiency (Bauman et al., 1985). Performance is the best indicator of a dairy cow’s well-being, and gains in productive efficiency over the last half-century are truly remarkable (Figure 1). Nevertheless, with each advance some have suggested cows are being pushed too far, thereby causing metabolic stress and compromising health and well-being. This concern was articulated over 50 years ago when Sir John Hammond pointed out that “considerable doubts have been expressed as to whether we are not now pressing high production in our farm animals too far, thereby undermining their constitution and so shortening their life” (Hammond, 1952). Hammond (1952) found no support for this opinion, and he concluded, “the physiological limits to intensive milk production are…only limited by our knowledge concerning the specific nutrients required for milk production.” Subsequent re-evaluations of the biological limits of production by Bauman et al. (1985), Knight et al. (2004), and Reynolds (2004) reached a similar conclusion. Indeed, an impressive