Heat Stress Considerations
Heat stress is a common occurrence in many areas during varying periods of the year. Management practices such as shade have obvious effects on the severity of heat stress. This and other considerations for dairy cattle were recently reviewed by West (2003). West (2003) summarized that the two most important effects on dairy cattle of heat stress are reduced DMI and efficiency of milk yield, which should also be transferrable to lactating dairy goats. It was suggested that ration formulation in light of heat stress should consider an increased nutrient density, altered nutrient requirements, avoidance of nutrient excesses, and maintenance of normal rumen function.
Based on historical production at a farm, a first step to changing diet composition in response to heat stress is to estimate the degree to which DMI is affected. For example, without heat stress DMI for a particular diet by a specific class of goat at a certain stage of production might be 4.5% BW. But with heat stress, DMI could be 10% less or 4.05% BW. Hence, the concentration of some nutrients in the diet might need to be increased to achieve the same level of production or to attain a nearly similar level.
One of the dietary characteristics that can be changed with heat stress is to increase the level of supplemental fat. Drackley (1999) provided a review in this area for dairy cattle. Without heat stress, although affected by numerous factors such as the fat source and type of dietary forage, a level of supplemental fat of about 3% of the diet is often most economical with basal diets having an ether extract or 'fat' level of approximately 3%. Certainly with heat stress a dietary level of supplemental fat of at least this should be employed, and perhaps levels slightly greater would be advantageous.
West (2003) suggested that the dietary total CP concentration should be changed only to the extent of meeting the requirement given the presumed reduction in DMI. However, dietary DIP and UIP concentrations may become of greater importance with reduced DMI due to heat stress, which causes an increased extent of ruminal CP degradation due to longer digesta residence in the rumen. It was stated that DIP as a percentage of total CP should be no more than 61% with heat stress. Hence, substitution of feedstuffs relatively high in UIP for ones lower (or relatively high in DIP) might be warranted. For such modifications, if using a commercial or custom concentate mixture, it is necessary to know exact levels of each ingredient.
West (2003) also indicated that heat stress may affect the optimum dietary cation to anion difference (DCAD), calculated as ((Na + K) - Cl) / 100. Although optimum DCAD have not been well established, with a database of dietary cattle experiments Hu and Murphy (2004) determined maximum milk yield and DMI at DCAD of 34 and 40 meq, respectively. With heat stress, changes in dietary levels of mineral sources to increase the DCAD above levels in non-heat stress periods may be beneficial.
Another common dietary change with heat stress in dairy cattle is to increase the concentrate level and decrease that of forage or roughage (Drackley et al. 2003). In addition to the greater digestibility of concentrates than forages, this can be beneficial because of the lower heat increment for concentrate. This is also one of the benefits of increasing the level of supplemental fat (lower heat increment than for both concentrates and forages). However, some lactation diets may already be high in concentrate (e.g., 50 to 60% of dietary DM), which may not allow for any or much of an increase in the concentrate level without incurring or increasing problems with rumen function. That is, relatively high dietary levels of concentrate, particularly with high-quality forages small in particle size that are ingested rapidly, can necessitate inclusion of ruminal buffers, most frequently sodium bicarbonate (typically up to 1% of dietary DM).
Drackley, J. K. 1999. New perspectives on energy values and supplementation levels of supplemental fats. Advances in Dairy Technology. Vol. 11. J. Kennelly (Editor). Proc. Western Canadian Dairy Seminar. pp 171-184. Univ. Alberta, Edmonton, AB, Canada. (available at www.wcds.afns.ualberta.ca/Proceedings/1999/chap16.htm).
Drackley, J. K., T. M. Cicela, and D. W. LaCount. 2003. Responses of primiparous and multiparous Holstein cows to additional energy from fat or concentrate during summer. J. Dairy Sci. 86:1306-1314.
Hu, W., and M. R. Murphy. 2004. Dietary catioin-anion difference effects on performance and acid-base status of lactating dairy cows: a meta-analysis. J. Dairy Sci. 87:2222-2239.
West, J. W. 2003. Effects of heat-stress on production in dairy cattle. J. Dairy Sci. 86:2131-2144.