September 5, 2017 at 3:32 p.m.
Research gleanings on heat stress, sugar addition to the diet
It will not be long and the real summer heat and humidity will be upon us. We had a sampling of it this month with some cows in some barns showing the first discomforts of heat stress. The best and most effective way to minimize heat stress is with fans and sprinklers. Adjustments in nutrition and feeding management can alleviate some of the negative effects on health and performance, but environmental modification is first and foremost in relieving heat stress on cows.
Immediate and residual effects of heat stress and restricted intake on milk protein and casein composition and energy metabolism. F. C. Cowley et al., J. Dairy Science 98:2356. (April 2015)
The effects of heat stress on dairy production arise from two separate causes: reduced voluntary feed intake, and the direct physiological and metabolic effects heat stress has on the cow. The aim of this Australian research was to separate the direct effects of heat stress from the secondary reduced feed intake effect and look at milk protein (casein) responses related to heat stress. To determine these effects, cows were housed in environmentally controlled chambers and subjected to heat stress (temperature-humidity index (THI) ~78) or kept in thermal neutral conditions with either ad lib or restricted feed intake.
A 20 percent reduction in milk production was observed in heat stressed cows compared to cows in thermal neutral conditions with ad lib intake. Heat stress cows averaged 6.3 pounds per day less milk than cows in thermal neutral conditions with restricted feed intake indicating factors beyond reduced feed intake are involved in the reduction of milk production during heat stress. Heat stress decreased milk protein and casein production and increased MUN more than reduced feed intake. They proposed heat stressed cows use muscle protein for energy resulting in more nitrogen (urea) in blood, which would account for increases in plasma and milk urea nitrogen.
Effect of core body temperature, time of day, and climate conditions on behavioral patterns of lactating dairy cows experiencing mild to moderate heat stress. J. D. Allen et al., J. Dairy Science 98:118. (January 2015)
Data used in this study came from farms in Arizona, California and Minnesota. The THI for Minnesota was 68 (70 degrees, 80 percent humidity) the same as California (73 degrees, 56 percent humidity), but lower than Arizona at 80 (91 degrees, 40 percent humidity). Arizona cows were subjected to more continuous 24-hour heat stress than cows in Minnesota or California. The Minnesota data was collected during the summer of 2009.
Heat-stressed cows spend more time standing. Arizona cows averaged 12.6 hours of standing per day compared to 11.4 and 11.3 hours for Minnesota and California cows, respectively. Peak standing period in all three locations was noon to 6 p.m., the hottest part of the day. When standing, more body surface is exposed for heat loss through water loss, radiating surface heat and air movement than when lying down. A core body temperature of 102 degrees was the pivotal point where above this temperature cows begin standing more and when lower than 102 degrees, cows would lie down more. In addition, as core body temperature increased, cows not only stood more, but they stood for longer periods. Standing limits blood flow to the udder compared to the lying position and as cows stand more, milk production is decreased. In addition, longer periods of standing increase the risk of lameness.
Both of these research reports support the need for good heat abatement measures (shade, fans, sprinklers, ample water) in the cow's environment. Diet modifications such as feeding highly digestible feeds and fiber sources along with fat addition helps reduce the heat load on cows, but cooling the cow directly is the primary means of reducing heat stress.
Effects of partial replacement of dietary starch from barley or corn with lactose on ruminal function, short-chain fatty acid absorption, nitrogen utilization, and production performance of dairy cows. G. E. Chibisa et al., J. Dairy Science 98 :2627. (April 2015)
This research from Canada addresses some misconceptions about feeding sugar and provides some guidelines for feeding whey or lactose to cows. Researchers replaced 6 percent of the ration DM from corn or barley in barley silage/alfalfa hay based lactation rations with dried whey permeate (DWP). Ration sugar content was either 3 percent (control) or 8 percent with added DWP. Starch content of the low sugar ration was 24.4 percent (DM basis) compared with 19.9 percent for the high sugar ration. The addition of DWP did not affect DM intake (average across all treatments, 64.7 pounds per day), milk production (89.6 pounds per day) or milk components (fat, 3.51 percent: protein, 3.34 percent). What DWP did was decrease rumen ammonia levels and increase rumen butyrate along with some other volatile fatty acids (VFA) in the rumen. However, sugar addition to the ration had no effect on rumen acetate and propionate production or rumen pH.
Sugar is rapidly fermented in the rumen (500 percent per hour) and as a result, many people expect rumen pH to decrease. In this study, similar to other studies, adding sugar did not decrease rumen pH. Based on this study and others, addition of sugar to lactation rations can potentially attenuate rumen acidosis. It is theorized the increase in rumen butyrate production from sugar feeding stimulates growth of rumen papillae allowing for increased absorption of VFA from the rumen. With less accumulation of VFA in the rumen, rumen pH does not decrease as much and therefore, the risk of acidosis from rapidly fermented feeds is reduced.
With the addition of lactose to the rations and the resulting increase in butyrate production, beta-hydroxy butyric acid (BHBA) levels in the blood increased. Cows fed sugar averaged about 0.6 mg/dL higher in BHBA than control (no added sugar) fed cows. Sugar fed cows had a 1.1 mmol/L BHBA level or just slightly below the ketosis threshold of 1.2 to 1.4 mmol/L.
Based on this study and earlier studies, up to 6 percent lactose can be used to replace starch in lactating dairy cow rations. As in this study, not all studies and farm experiences with sugar additions to rations have improved milk production. However, benefits of enhanced rumen fermentation are commonly seen with sugar addition to rations. Increased production of butyrate in the rumen and the associated increased rumen papillae growth and absorption of VFA, may also have benefits during heat stress when VFA tend to accumulate in the rumen.
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