ISSUE FOCUS FEED & ADDITIVE MAGAZINE July 2022 37 • Nitrogen balance. In conditions of extreme heat cows might experience negative nitrogen balance due to reduced feed intake and higher excretion of nitrogen. This results in reduced milk production. The target should be improving nitrogen balance but not simply by elevated dietary crude protein (CP) levels. There needs to be a good balance of rumen degradable and undegradable protein. Excess degradable protein can release too much rumen ammonia that cannot be fully captured by rumen microbes and the liver can struggle to deal with this. A study during hot temperatures showed that cows fed diets with 16% CP with a low degradable protein content had 1.5 kg higher milk yield compared to animals fed a diet with 18% CP which was highly degradable (Higginbotham et al., 1989). Correct nitrogen balance in the rumen can be achieved by introducing ingredients with lower protein degradability: brewers grains, distilleries dried grains and soluble (DDGS). When feeding some co-products in fresh moist form, spoilage can be an issue in hotter climates and needs to be taken into account. Spoiled/mouldy feed should be carefully separated and never fed to animals. • Electrolyte balance. The Electrolyte Balance (EB) of the ration is considered as another factor that can help to mitigate the harmful effect of heat stress in milking cows. The final dietary EB depends on the content of ions Na+, K+, Mg2+, Cl-, S2- & P2- and can be manipulated by the restriction of one type of ions and supplementation of of desirable electrolyte minerals. It is recommended to maintain the following concentrations of minerals as % DM: К – 1.3-1.5; Na ≥ 0.5; Mg ≥ 0.35 & chlorine ≤ 0.35. Higher levels of K and Na should compensate for greater losses due to sweating compared to chlorine. Excessive chlorine levels may depress DMI and milk yield, respectively. The elevated levels of K can suppress Mg absorption in the rumen, requiring a higher Mg intake. In conditions of heat stress increasing EB from 120 to 460 mEq/kg DM has a positive effect on appetite and DMI of milking cows (West et al, 1992). It is recommended to maintain EB close to 300 mEq/kg DM. Sodium bicarbonate helps in keeping desired dietary Na levels and EB without excessive chlorine content. • Mineral buffer additives. Improving the buffer capacity of the diet can contribute to maintaining normal pH in the rumen, milk production and butter fat during extreme ambient temperature. • Live yeast (Saccharomyces cerevisiae) supplementation. Heat-stressed cows suffer decreased rumination (Collier et al., 2006) and reduced amplitude and frequency of rumen contractions (Bernabucci, 2012). Decreased rumination results in reduced saliva flow and buffering capacity is also reduced due to increased CO2 loss via panting. Furthermore, a decreased rumen pH impairs fibre digestion efficiency, with rumen fibrolytic bacteria being most affected when rumen pH drops below 6.0. The intraruminal temperature may affect rumen metabolism (Gengler et al., 1970). These authors found that an increase in intraruminal temperature is related to a decrease in volatile fatty acid (VFA) production and a shift in their profile with a significant decrease in the acetate to propionate ratio. To counteract these negative effects in ruminal efficiency and metabolism and to maintain health status, fertility and performance, live yeast has successfully been used in dairy cow diets during periods of heat stress. As live yeast removes oxygen in the rumen, it improves the conditions for growth of those bacteria that convert lactic acid to propionic acid which is a major energy source for the ruminant. In addition, live yeast effectively competes with starch degrading bacteria for sugars and reduces their growth and subsequent lactic acid production. Thus, live yeast can help to prevent accumulation of lactic acid in the rumen, helping to regulate the rumen pH and limit the risk of both clinical and subclinical acidosis in animals experiencing HS. In the summer of 2019 during the heatwave affecting Europe, in those herds which were treated with a double dose of live yeast (100x109 CFU/head/day) no significant drop in milk yield was seen and strong daily fluctuations were not experienced.
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