Context: In temperate climates, multiparous cows are more susceptible to heat stress (HS) than are primiparous cows. However, these differences in susceptibility may vary in warm environments, where the overall production level is lower, cow size is smaller, and adaptation to HS increases. Aim: The different effects of HS and its alleviation on primiparous and multiparous cows was tested in crossbred cows (3/4 Holstein × 1/4 Brahman) in a warm environment. Method: Twelve cows during the rainy season (August–September 2014) and 12 cows during the dry season (March–April 2015; six primiparous, six multiparous) were monitored for rectal temperature and respiration rate, indicators of HS, and milk yield (MY) in a dairy farm in El Salvador. Ambient temperature and relative humidity were recorded hourly to estimate temperature–humidity index throughout the day. During the rainy season, all cows were housed in a pen without cooling treatment. During the dry season, fans and sprinklers were installed in the pen and provided cooling to the herd in two 15-day periods, alternated with two periods without cooling (Control). Key results: Primiparous cows were more susceptible to HS. Cooling alleviated HS, resulting in an increased feed intake and MY, with a greater impact on primiparous cows. During the cooling treatment, HS parameters increased overnight after the last cooling cycle of the day. This indicated that, despite the cooling treatment, cows still suffered from HS outside the hottest hours of the day. Furthermore, despite a higher temperature–humidity index during the rainy season, rectal temperature and MY remained similar in both the dry and the rainy season for cows without cooling, indicating that animals effectively lower their body temperature by increasing their respiration rate. Conclusions: The cooling only partially helped alleviate HS in both multiparous and primiparous cows, with the effects being greater on the latter. However, the increments in MY were proportional to the increases in feed intake, indicating that the efficiency of feed utilisation was likely to be not improved. Implications: More attention should be given to the susceptibility of primiparous cows to HS, as well as to the adjustment of cooling protocols towards alleviating HS in early morning and late afternoon hours.

The present study aims at evaluating whether current semimechanistic models developed for temperate cattle systems can be adopted for cattle under (sub‐) tropical husbandry systems to adequately (accurately and precisely) predict total nitrogen (TN), urine nitrogen (UN), faecal nitrogen (FN) excretion and its partition into different FN fractions. Selected models were built based on the feeding recommendations for ruminants of the British (Model A), German (Model G) and French (INRA; Model I) system. Model evaluation was conducted using eight nitrogen balance studies performed in El Salvador, Kenya and Peru (n = 392 individual observations including lactating cows, heifers and steers). Concordance correlation coefficient, root mean square errors (RMSE), and mean biases were estimated to evaluate the models' adequacy in predicting nitrogen excretion. Input variables causing greatest variation in nitrogen excretion prediction were identified by a sensitivity analysis and adjusted. Model G was able to adequately (i.e., RMSE of <25% of observed mean, systematic error of 56%), and TN (RMSE = 22%) excretion, by Model I.

This study evaluated the impact of incorporating citrus pulp (CiP) into the rations of high-producing dairy cows under tropical conditions. Eighteen lactating dairy cows were assigned to two dietary treatments: corn meal (CM) or CiP as the main energy source. Dairy cows were allocated to a crossover design comprising two 21-day periods. The rations were formulated to be isocaloric and isonitrogenous. Replacing CM with CiP reduced the intakes of dry matter (19.9 vs. 19.5 kg/d), organic matter (17.9 vs. 17.4 kg/d), digestible organic matter (12.3 vs. 11.7 kg/d), and crude protein (3.43 vs. 3.35 kg/d), while increasing neutral detergent fibre intake (7.39 vs. 7.63 kg/d). Apparent total tract digestibility decreased for all nutrients, including DM, OM, CP, NDF, and ADF, when CiP replaced CM. Milk production was lower in cows fed CiP than in those fed CM (23.7 vs. 22.7 kg/d), although milk feed efficiency (milk/DMI) was not different. An economic analysis showed that cows fed CM had higher milk gross income and income over feed cost. These results suggest that the partial replacement (60%) of CM with CiP may negatively affect feed intake, nutrient digestibility, milk production, and profitability in dairy cows in tropical regions.