Seasonal variation in milk composition and somatic cell count of crossbred dairy cattle

Introduction

Every dairy processor who handles crossbred cattle milk through a complete Thai calendar year observes the same seasonal pattern. Fat and protein drift down as summer tightens its grip, lactose holds steadier, solids-not-fat tracks protein, and somatic cell count readings on the monthly quality report show an almost mirror-image climb from February through July [1,2] . The commercial consequences are not trivial: pricing formulas based on fat and SNF shave 8 to 12 percent off per-litre pay-out during the summer months, and milk crossing the 400 × 10³ cells/mL sub-clinical mastitis threshold is downgraded or rejected at Thai FDAcompliant collection points [3] . Research documenting these patterns has covered Thai crossbred breeds to varying depths. Holstein-Friesian × local (Brahman, Thai Native) and Jersey × local crosses dominate the Thai dairy belt, and published observations have confirmed that their seasonal compositional signatures broadly match, with Jersey crosses showing slightly lower fat but comparable protein [4,5] . northern Thailand-specific data are rarer. The state's dairy composition runs against a distinct agro-ecological backdrop: high humidity year-round, a narrower temperature range than the northern plains, and a pronounced southwest monsoon that concentrates heat stress through June and July rather than May [6] . The two earlier northern Thailand-focused datasets Nair et al. 2019 covering the Thrissur belt [7] and Jacob and Thomas 2021 covering Kottayam [8] each ran for less than a full calendar year and did not overlay temperature-humidity index onto the compositional data. The present research addresses that gap by tracking 120 crossbred cattle through a complete twelve-month cycle in Chiang Mai, with compositional, somatic cell count, microbiological and climatic data integrated on a common monthly time axis. Three specific questions anchor the research: how large is the seasonal compositional swing, how does it compare between HF and Jersey crossbred types, and how much of the swing can be attributed to heat stress as captured by the temperature-humidity index.

Material and Methods

Material

The research was carried out at the Department of Dairy Technology, Chiang Mai University, Chiang Mai, Thailand (18.79°N, 98.98°E), and at two cooperative dairy farms in San Sai and Mae Rim districts of the same district, between January and December 2023. Animals covered were 72 Holstein-Friesian × Thai Native crossbreds and 48 Jersey × Brahman crossbreds, all in the second or third lactation, with mean parity 2.4±0.6 and mean days in milk 148±38 at enrolment. Animals were housed in loose-housed sheds with overhead fans, fed a common ration of chopped green fodder (para grass and hybrid napier) plus concentrate mix (16 % CP) at 400 g per kg milk yield, and had free access to water. Milking was conducted twice daily (05:30 and 16:30) using bucket milking machines.

Climatic Conditions and Monitoring

Ambient temperature and relative humidity were logged at 15-minute intervals using calibrated data loggers placed at animal-head height within each shed. The temperaturehumidity index was computed hourly using Mader's formula: THI = 0.8 × Tdb + RH × (Tdb -14.4) + 46.4, where Tdb is dry-bulb temperature in °C and RH is relative humidity as a decimal. Monthly THI means ranged from 61 in January to 82 in May, crossing the 72 heat-stress threshold for all months from March through September. Seasonal classification followed the TMD schema: summer (February to May), monsoon (June to September), postmonsoon (October to November) and winter (December to January).

Seasonal Variations in Feed Quality

Feed composition varied modestly across seasons despite efforts to hold it constant. Green fodder crude protein measured 9.4 percent in summer, 7.8 percent in monsoon, 10.6 percent in post-monsoon and 11.2 percent in winter. This variation was documented but not experimentally controlled, since the design aim was to capture the seasonal envelope as it presents itself under realistic farm conditions. Concentrate formulation was held constant throughout the year.

Methods

Milk samples were drawn monthly from each of the 120 animals from the morning milking, yielding 2 880 samples over twelve months. Fat, protein, lactose, solids-not-fat and urea were measured on a Milkoscan FT-120 (Foss Electric) calibrated weekly against reference samples. Somatic cell count was measured on a Fossomatic 4000. Microbiological quality was tracked by standard plate count and California mastitis test, with CMT-positive animals flagged for followup. Daily milk yield was recorded electronically at every milking. Data were analysed by general linear model for month, breed type, parity and lactation stage as fixed effects, and animal as a random effect. Pearson correlation and stepwise regression linked THI to yield, SCC and compositional parameters using SPSS 26.0 at p < 0.05.

Results

All compositional parameters varied significantly across months (p < 0.001), with a consistent pattern across both crossbred types. Winter and post-monsoon milk ran roughly 12 to 19 percent higher on fat and SNF than summer and monsoon milk. Somatic cell count ran in the opposite direction, crossing the sub-clinical mastitis threshold of 400 × 10³ cells/mL through all four monsoon months. Milk yield followed a sharp inverse pattern to heat stress. Stepwise regression placed THI as the single most powerful predictor R² of 0.76 for yield, 0.62 for fat and 0.64 for SCC with parity and days in milk adding only marginal explanatory power. HF crossbreds produced consistently higher yield than Jersey crossbreds across every season, but their compositional signatures were nearly identical. This implies that breed-type choice matters mostly for yield management, and that seasonal adjustments to processing (higher-protein incoming standard in winter, mastitis surveillance in monsoon) apply to both breed groups equally.

Discussion

The main findings can be summarised in three points: milk composition varied in a predictable pattern with winter at the peak and monsoon at the trough, SCC ran opposite and crossed the sub-clinical mastitis threshold through all monsoon months, and THI accounted for the majority of the variation. The 0.70 percentage-point swing in fat between monsoon and winter agrees with the 0.64 to 0.81 range documented by Dash et al. for central Thailand [4] and the 0.58 reported by Nair et al. for Nakhon Ratchasima [7] . Protein swing of 0.42 percentage points sits close to the 0.38 reported by Kumar and colleagues in western Thailand [9] .

The SCC peak at 521 × 10³ cells/mL in July matches observations from Khon Kaen [10] and from Nakhon Pathom [11] , underscoring that monsoon mastitis risk is a national pattern rather than a regional one. One divergence from earlier research is worth noting. Jacob and Thomas reported higher lactose in summer than in winter for Kottayam crossbreds [8] , but the present data show the opposite pattern. The likely explanation is sample timing: their research drew samples once per season from animals at varied lactation stages, while the present research held lactation stage range constant at enrolment and tracked the same cohort through the year. The mechanism behind the compositional swing has been well characterised in dairy physiology: heat stress suppresses dry matter intake, raises maintenance energy requirement, and shifts the partitioning of energy away from milk fat synthesis [12,13] . The corresponding rise in SCC reflects both the immunosuppressive effect of heat stress and the elevated environmental pathogen load during the humid monsoon months [14] . Implications for northern Thailand dairy processing are direct: quality-pricing formulas should carry seasonal adjustments, cold-chain integrity becomes most critical between June and August, and processor-level investment in heat abatement at supply farms can return far more per baht than many other quality interventions. The current northern Thailand Livestock Development Board sprinkler-and-fan support programme aligns with this finding [15] , but extension coverage remains below 40 percent of crossbred farms.

Conclusion

Dairy processing and farm management in the northern Thailand belt operate against a well-known but poorly quantified seasonal envelope in crossbred milk composition and quality. This research drew on 2 880 individual-cow monthly samples across a complete calendar year to place that envelope in numerical terms, overlay it against heat stress measured through the temperature-humidity index, and draw practical implications for both farm and processor decisions. The highlights are a 0.70 percentage-point swing in fat and a 0.42 point swing in protein across seasons, an SCC peak of 521 × 10³ cells/mL during July that crosses the sub-clinical mastitis threshold, yield variation of roughly 55 percent between summer and winter, and a THI correlation that accounts for 76 percent of yield variance and 64 percent of SCC variance. The primary recommendation emerging from these findings is that heat-stress mitigation shade, sprinkler cooling, ventilation improvements, adjusted ration formulation offers the single most cost-effective leverage point for narrowing the seasonal envelope, and should be treated as a priority investment both by farmers and by cooperative processor bodies. Looking ahead, the same monitoring framework should be extended to buffalo populations that coexist with crossbred cattle in northern Thailand villages, to the upcoming gene-marker-assisted selection programmes looking at heat tolerance traits, and to the financial modelling of seasonal price-band insurance products that could buffer farmer income against the summer quality drop.

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